The occurrence of multiple genetically and phenotypically distinct forms in a single interbreeding population, known as polymorphism, represents a long-standing puzzle in evolutionary biology. Several mechanisms, both selective and stochastic, have been proposed to account for the maintenance of such diversity. Nevertheless, although our knowledge about how these mechanisms might operate has increased substantially in recent years, the specific role that they play in the maintenance of polymorphisms in natural populations remains to be determined. In particular, negative frequency-dependent selection, where rare morphs have a fitness advantage over common morphs, has frequently been suggested to explain the occurrence of genetic variation, but conclusive evidence for its importance in natural populations has yet to be obtained. In this thesis, I investigated the maintenance of a female-limited colour polymorphism in the crab spider Synema globosum, and the role that negative frequency-dependent selection could have in this process. First, I described the natural history of S. globosum, which was previously unknown. Then, I looked at the nature of the colour polymorphism and investigated its mode of inheritence. Finally, I carried out a series of experiments to examine the potential role of negative frequency-dependent selection resulting from interactions with prey, and between potential mates, in the maintenance of the polymorphism. S. globosum was shown to be an abundant species in the study area and, apart from its striking polymorphism, it exhibits several interesting characteristics that would make it a useful model species for studies of behaviour, ecology, and evolution. The female morphs were shown to be discrete and genetically determined. No evidence for directional selection favouring one particular morph, or for geographic variation in selection, was found. Field experiments revealed an effect of a previous negative experience with a particular colour morph on subsequent responses of honeybees (Apis mellifera), one of S. globosum’s main prey, to the presence of spiders on flowers. This result provided support for the hypothesis that such interactions generate negative frequency-dependent selection. Similarly, mating experiments provided some evidence that reduced harassment of less frequently encountered female morphs by male S. globosum could also generate negative frequency-dependent selection. This work adds to a growing body research that has increased our understanding on the mechanisms that maintain diversity in nature and establishes the basis for future studies to investigate the exact ecological explanation for the observed phenotypic variation in S. globosum.

Black widow spider venom (BWSV) contains high molecular weight proteins called latrotoxins (LTX) that induce catastrophic neurotransmitter release from nerve terminals, and one toxin, α-latrotoxin, is known to bind with high affinity to three neural proteins in mammals, including latrophilin (lat-1) a member of the class B family of G-protein coupled receptors. We have established C.elegans as a model organism to study the function of the binding protein, lat-1 and its role in regulating neurotransmitter release by latrotoxins. However, a lat-1-knockout worm is required for determining the function of the lat-1 gene. The lat-1(ok1465) allele has a deletion of the lat-1 gene, and ~95-98% of lat-1(ok1465) homozygous worms arrest or die before adulthood, with only ~2-5 adult offspring per animal. Micro-injection of the B0457 cosmid, that contains the full sequence of the lat-1 gene, or the lat-1a cDNA rescued the lethality of the lat-1 worms, thereby showing that lat-1 gene is responsible for the developmental lethality in these worms. Expression of the marker, GFP, under the control of the lat-1 promoter showed that there was expression of GFP during epithelial morphogenesis, and strong expression in the gut from the three-fold stage through to larval stages. The concordance between the site of expression of lat-1::gfp, with the sites of embryonic defects (epithelial enclosure defects; defective attachement of gut) in lat-1(ok1465) animals, provides further evidence that lat-1 is essential for embryonic and larval development. Deletion mutants of lat-1a were constructed to examine the role of domains of this protein. Deletion of sequences after the 4xCys domain of lat-1a did not affect the ability to rescue lethality in the lat-1 worm, while deletion of the C-terminus to the seven transmembrane domain impaired the ability of lat-1a to rescue lat-1 worms, and further deletion of six of seven transmembrane domains (the TM1 construct) yielded a construct that was unable to rescue lat-1 worms. These data suggest an important role for intracellular sequences and seven transmembrane in lat-1a signalling. It was proposed that TM1 could decoy ligand, without causing intracellular signalling. In agreement, the TM1 construct caused a mild phenocopy of the lat-1(ok1465) mutant in wild-type worms, whereas full-length, or non-ligand binding variants of lat-1a caused no such effect. To investigate the putative ligand-binding domain of lat-1a, deletion of residues 62-147 (ΔGBL), 62-250 (ΔHRM) and 62-487 (ΔN) was investigated; while the ΔN construct was incapable of rescuing lat-1(ok1465) worms, deletion of ΔGBL had a minor effect on the ability of lat-1 to rescue the null worms, while ΔHRM had a more marked effect. These data are consistent with a model whereby residues 147-487 are required for ligand binding, and the seventransmembrane and intracellular domains transmit a signal to the inside of the cell. Combined latrotoxins was highly toxic to wild-type C.elegans (LD50 ~4ng/ml), whereas the lat-1 worms were highly (>105-fold) resistant to combined latrotoxins. Lat-1 worms that were transgenic for B0457cosmid, or lat-1a cDNA, were as sensitive to combined latrotoxins as wildtype worm. Truncation of the C-terminus of lat-1a to TM1 yielded worms that had 105-fold resistance to combined latrotoxins, compared to wild-type; thus the intracellular domain of lat-1 is required for mediating combined latrotoxins toxicity. The deletion of galatactose-binding lectin (ΔGBL) in N-terminus lat-1a was sensitive as wild-type, but deletion of hormone receptor motif (ΔHRM) in N-terminus lat-1a showed a reduced sensitivity to combined latrotoxins by ~105-fold. These data showed presence of lat-1 gene was responsible for the rescue of lat-1 worms or toxicity of combined latrotoxins in lat-1 worms, and the absence of lat-1 gene was responsible for the lethality of lat-1 worms and resistance to combined latrotoxins in lat-1 worms.

The Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor, which mediates the potent toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds. AhR is regulated by the ligand-binding domain (LBD) of the AhR, and so determining how the binding of ligand activates AhR is of considerable interest. However, there are no structural data on mammalian AhR LBDs, and expression of the mouse AhR LBD in E. coli yields insoluble protein. Expression in more complex systems, such as insect cells (Spodoptera frugiperda), yields soluble AhR LBD, but only ~10% of the total protein is in a ligand-binding competent form. In order to address the structure of the AhR LBD, we have used a model system. There is good amino acid sequence similarity between human AhR and C. elegans AhR (CeAhR). We have investigated whether the three dimensional structure of CeAhR LBD will help in understanding this structure in mammals. CeAhR LBD was cloned into the vector pRSET to give histidine-tagged protein. The clones were then transformed into E. coli BL21(DE3) or Arctic Express strains, followed by induction with IPTG. Bacteria were lysed and 100000g supernatants were prepared. Proteins were purified by Ni2+ affinity chromatography. Expression of recombinant proteins in the bacterial system revealed that the induced protein from the pRSET.CeAhR LBD construct was ~29 kDa, as predicted. Large amounts of these proteins were produced (~5-10% of total bacterial protein) and the vast majority was insoluble. However, on preparation of a 100000g supernatant, the samples yielded small amounts of soluble CeAhR LBD fusion protein. This is in contrast to results obtained with mouse AhR LBD, which yielded no detectable protein in a 100000g supernatant. The CeAhR LBD proteins were successfully purified by affinity chromatography and were obtained in good yield from the original cytosols. However, the yield of soluble AhR fusion protein was ~100 microgrammes of protein per litre of BL21(DE3) bacterial culture. The experiment was repeated using Arctic Express bacteria, which have a constitutively expressed chaperonin, and express at 12°C. However, the yield of protein was similar, at ~100 microgrammes of protein per litre. Thus the CeAhR LBD yields soluble protein in a bacterial expression system, but the levels of expression are too low to enable this protein to be purified for use in structural studies. Trials to express CeAhR LBD in transgenic C. elegans and Pichia pastoris yielded no soluble protein. The research moved to look for ligands for CeAhR by using a lethality test with C. elegans in vivo studies. The results showed that TCDD and AZ1c (from AstraZeneca) affect the wild type C. elegans, but without killing them. Repeating this test on AhR null animals showed that the effects were abolished. Thus the CeAhR is a receptor that appears to bind TCDD and AZ1c, albeit weakly, contrary to previous reports in the literature.

Uncoupling protein 1 (UCP1) is essential for non-shivering thermogenesis in brown adipose tissue (BAT) by dissipating proton-motive force to stimulate maximum mitochondrial respiration. cAMP-dependent protein kinase induction of UCP1, as well as the PPAR coactivator1α (PGC1α), is a typical characteristic in BAT but not in white adipose tissue (WAT). Previous work demonstrated that the overexpression of CCAAT/Enhancer Binding Protein β (C/EBPβ) could rescue the cAMP-induced PGC1α and UCP1 expression in white preadipocytes 3T3-L1 cell line, indicating a key regulatory role of C/EBPβ in brown adipogenesis. The overall aim of this study was to examine the role of C/EBPβ overexpression in regulating the transcription of UCP1 in 3T3-L1 white preadipocytes to transform them to a more brown-like cell phenotype. Tetracycline inducible (Tet on) lentiviral, adipose-specific expression vectors for overexpressing C/EBPβ (or the control Luciferase-GFP) gene were constructed with the pLenti6 lentiviral vector backbone with TRE tight and rtTA advance regulatory elements. In the absence of doxycycline there was low basal expression from the vectors and a dose-dependent, doxycycline-induced transient, adipose-specific overexpression was observed in 3T3-L1 preadipocytes. Transduction of the pLenti6 positive control luciferase-RFP vector was successfully achieved but the C/EBPβ or LucGFP vectors constructed failed to produce highly infectious lentiviral particles, possibly due to the large size of insert which challenged the limit of the pLenti6 vector backbone. Therefore a stable inducible, adipose-specific, 3T3-L1 line overexpressing C/EBPβ was not achieved. Transient overexpression of C/EBPβ and PRDM16 significantly increased the transcriptional activity of UCP1 promoter in the presence of forskolin in 3T3-L1 cells without stimulating PGC1α promoter activity, implying a PGC1α-independent manner of activating UCP1 transcription in 3T3-L1s. C/EBPβ overexpression alone activated the PGC1α promoter in HIB-1B and Cos7 cells but not in 3T3-L1 cells, indicating the lack of some activators in 3T3-L1 or a potential 3T3-L1 specific repressive mechanism. Co-overexpression of C/EBPβ and PPARγ in 3T3-L1 markedly stimulated the PGC1α promoter in response to rosiglitazone and increased the UCP1 promoter activity in the presence of rosiglitazone and forskolin. This result suggests that PPARγ could make up for the lack of activator or release the 3T3-L1 repressive mechanism and mediated a PGC1α-dependent manner of activating UCP1 together with C/EBPβ in 3T3-L1 cells. Further studies demonstrated that both CRE and PPRE elements were indispensible in this PGC1α-dependent pathway of activating UCP1 promoter in 3T3-L1 cells. The results suggest that C/EBPβ and PPARγ cooperate to induce UCP1 expression in 3T3-L1 cells stimulated by PPARγ agonist and cAMP.

How DNA repair proteins search and recognise the rare sites of damage from the massive numbers of normal DNA remains poorly understood. FapydG (2,6-diamino-4-hydroxy-5-formamidopyrimidine) is one of the most prevalent guanine derived lesions involving opening of the imidazole ring. It is typically repaired by formamidopyrimidine-DNA glycosylase (Fpg) as an initial step in base excision repair; if not repaired, the lesion generates a G: C -+ T: A transversion. Unfortunately, studies on the recognition of FapydG have been hindered by difficulties to synthesise and incorporate the FapydG residue into a DNA duplex. Crystal structures of Fpg-DNA complexes have demonstrated three common recognition events: the protein specifically binding to the extrahelical lesion, bending DNA centred on the damaged base, and flipping the damage into the pocket. Thus, molecular modelling and dynamics simulation have been used to gather dynamical information of those recognition events for damaged and undamaged DNA. The simulations were initially performed when FapydG or G occurs in several dodecamer B-DNA sequences in aqueous solution, then inside the lesion-recognition pocket of Fpg, and during the flipping pathway from the helical stack to an extrahelical position. The influence of the damage on DNA stability and flexibility was first investigated. Energetic analysis revealed that damage to DNA does appear to destabilise the duplex. DNA curvature analysis and a novel combined method of the principal component analysis (PCA) and the Mahalanobis distance (DM) indicated that damaged DNA can adopt the observed protein-bound conformation with lower energetic penalties than its normal counterpart. Results of these studies have provided the validation of DNA bending enhancement by the FapydG lesion. It also suggested that intrinsic DNA bending could be a principal element of how the repair protein locates the lesion from vast expanse of normal bases. Considering the specific recognition of FapydG by Fpg, the aF-/39 loop of the Fpg enzyme may function as a gatekeeping to accommodate the lesion while denying the normal base. Remarkably fluctuating movement of the flipped G residue and the aF-ß9 loop is due to the formation of the non-specific Fpg/G complex with a lower binding energy by 8.4 kcal/mol compared to the specific Fpg/FapydG complex. Free-energy profiles for both damaged and undamaged base flipping were generated from the umbrella sampling simulations and the Weight Histogram Analysis Method (WHAM). An energy barrier for flipping the damage out from the helix is 2.7 kcal/mol higher than its equivalent G and the lesion is highly stabilised inside the pocket. In contrast, G flipping seems to be rapidly rotated out and into the duplex without the formation of a specific complex. These studies could unravel a potentially comprehensive process of the repair protein to find and recognise the lesion through the slow kinetic pathway in which the more deformable damaged DNA is initially located by the protein; the protein subsequently compresses the duplex into an appropriate angle and direction to form a specific protein-DNA complex prior to being flipped and repaired.

Porcine somatic cell nuclear transfer (SCNT) offers new opportunities for fundamental science, medicine and agriculture. Ten countries or regions have developed the technology of porcine cloning and at least 42 groups have succeeded in producing piglets. Although successful, the efficiencies and reproducibility of porcine SCNT are extremely variable. The technique of SCNT involves multiple steps, each of which can affect subsequent development. In particular, the synchrony of maturation, biochemical status of the matured oocytes and methods of parthenogenetic activation are thought to be major factors influencing development. The objectives of these studies were to optimise these steps to produce an efficient and reproducible method of porcine SCNT. Cycloheximide (CHX) and cyclic AMP (cAMP) have been reported to maintain oocytes at the germinal vesicle (GV) stage and synchronise subsequent maturation. The effectiveness of these two treatments in inducing synchronisation was evaluated. Then nuclear status of oocytes was examined by aceto-orcein staining after release from CHX and cAMP at 0 h, 12 h, 22 h, 28 h, 36 h and 44 h. Data was analysed by chi-square test. At 28 h, 78.89%, 77.78% and 73.33% of control, CHX and cAMP oocytes reached metaphase of the first meiotic division (MI) respectively (p > 0.05). At 36 h, the frequency of oocytes at metaphase of the second meiotic division (MII) of the cAMP group (8.64%) was significantly lower than those of control and CHX groups (74.29% and 47.31 %, respectively; p<0.00 1). At 44 h, there was no difference between control and cAMP groups (91 % and 83.72%, respectively; p>0.05), however, the proportion of MII oocytes in the CHX group (56.57%) was lower (p < 0.05). The results demonstrate that CAMP is more effective than CHX in synchronising porcine oocyte maturation with oocytes reaching MII during a shorter time window. Parthenogenetic development of porcine oocytes synchronised by CHX and cAMP treatments was compared by the frequency of cleavage at 48 h post onset of activation (hpa) and blastocyst formation at 168 hpa. No significant differences were observed in the frequency of cleavage (96.7 ± 2.1%, 81.4 ± 11.6% and 84.5 ± 5.7%, respectively), development to blastocyst (28.3 ± 11.4%, 27.1 ± 5.7% and 32.8 ± 5.3%, respectively) between control, CHX or cAMP treated oocytes respectively (chi-square test, P>0.05). However, total cell number was significantly higher in CHX group than cAMP group (42.7 ± 4.1 and 31.8 ± 2.0, respectively; t-test, P<0.05). The results demonstrate that synchronisation of porcine oocytes by treatment with either CHX or cAMP does not affect subsequent parthenogenetic development as judged by blastocyst formation, however the total cell numbers after CHX treatment were higher than those after cAMP treatment (P < 0.05). cAMP was selected to synchronise porcine oocytes because maturation to MII was more controlled and occurred over shorter period. The meiotic progression of cAMP treated oocytes was recorded at 38-44 h post onset of maturation (hpm) with telophase of the first meiotic division (TI; 35.6 ± 12.8%) peaking at 38 hpm, hence 36 -38 hpm chosen as a time window for TI enucleation. The percentage of TI porcine oocytes successfully enucleated was 98.1 ± 1.9%. Caffeine (5,10 or 20 mM) had no significant effects on either maturation promoting factor (MPF) or mitogen-activated protein kinase (MAPK) activities of oocytes of TI and early MII arrested oocytes after 6 hours (t-test, P>0.05). Although MPF and MAPK activities in TI enucleated oocytes at 44 hpm were higher than those at 38hpm reaching maximum level at 44 hpm (two-way ANOVA, P<0.05), 5 mM caffeine did not change either of MPF and MAPK activities in TI enucleated oocytes (two-way ANOVA, P>0.05). Finally, development to blastocyst stage of SCNT embryos using TI enucleated oocytes treated with 5 mM caffeine was recorded. The frequency of blastocyst formation obtained was 8.8 ± 0.7 % with average total cell number 29.7 ± 0.9. In conclusion, these studies have optimised several steps for porcine SCNT and produced porcine SCNT embryos using a homogenous population of porcine oocytes enucleated at earlier stages (TI stage) and treated with caffeine. These studies, along with further research may aid in the design of more successful methods of porcine somatic cell cloning.

Two new transcription factor binding sites have been identified within the phosphoglycerate kinase (PGK) gene promoter in the yeast Saccharomyces cerevisiae. The binding sites are upstream of the previously defined UAS, and are bound in vitro by the multifunctional transcription factors Reb1p and Cpf1p. A deletion of the Reb1p binding site was made from a PGK gene construct on a multicopy plasmid, and also targeted to the chromosomal copy of PGK. Deletions of the Rap1p and Abf1p binding sites in the UAS were also targeted to the chromosome. Analysis of RNA from the chromosomal deletion strains confirmed the central role of Rap1p in the activation of transcription from PGK. Reb1p and Abf1p were also found to be important for transcriptional activation. This is in contrast to results from experiments using multicopy plasmids carrying Reb1p or Abf1p binding site deletions from PGK. In this situation, neither the Reb1p site, nor the Abf1p site, plays a role in transcriptional activation. A role for Cpflp at the PGK promoter was examined using a cpfl null strain of yeast. Northern blot analysis was used to assay transcription from the chromosomal PGK gene in the absence of Cpf1p, and also transcription from a multicopy plasmid carrying the wild type PGK gene in the cpf1- background. In both cases, the absence of Cpf1p was found to have very little effect on the level of transcription. In addition, a role for the potential yATF binding site at the 3' end of the PGK UAS was investigated. Oligonucleotides containing this sequence were inserted upstream of a minimal promoter, and levels of a β-galactosidase reporter were assayed. No activation over the basal level was observed. A deletion of the potential yATF binding site from the UAS was made from a multicopy plasmid construct, and also from the chromosomal locus. Transcription from the deleted constructs was found to be no different from transcription from the wild type gene. Finally, DNA sequences which are able to complement the C-terminus functions of Rap1p were identified. A yeast genomic library was generated downstream of the N-terminus and DNA binding domain of Rap1p. This library was transformed into a rap1ts strain of yeast to look for complementation of the ts phenotype. Transformants which grew at the non-permissive temperature were obtained. Results from the analysis of the DNA sequences in these transformants are presented.

Sox2 is one of the earliest known transcription factors to be expressed during development of the nervous system (Rex et al., 1997; Silvia Brunelli, 2003; Wang et al., 2006b; Dee et al., 2008). Ectodermal cells expressing Sox2 have the potential to differentiate into nerve cells. Cells expressing Sox2 are specified to a neural fate during neural induction. Sox2 belongs to the SoxB1 family, comprising Sox1, Sox2 and Sox3, which are generally considered to activate specific target genes, whereas, the SoxB2 group, Sox14 and Sox21, act as transcriptional repressors (Uchikawa, Kamachi, & Kondoh, 1999). However, Sox2 has also been demonstrated to act as a repressor (Kopp et al., 2008) which implies that Sox2 could have a dual-function in vivo. Previous studies indicated that the HMG box-containing protein, Tcf/Lef, interacts with the transcriptional co-repressor, Groucho (Helen Brantjes, 2001). We therefore set out to determine if interaction with the Groucho co-repressor could also explain the repressor ability of Sox2. In this study, we have examined the interaction between Sox2 and Groucho using nuclear translocation, yeast-two-hybrid and co-immunoprecipitation assays. The data suggest that Sox2 interacts with Groucho through a C-terminal, engrailed-like motif. The effect of Groucho on Sox2 function was measured using a luciferase reporter assay. The transcriptional activation activity of Sox2 was repressed after co-expressing with Groucho. To address the biological function of Sox2-Groucho interaction, a loss-of-repressor-function mutant of Sox2 was created by point mutating the essential engrailed-like motif. Analysis in Zebrafish embryos indicated that the of loss-of-repressor-function mutant of Sox2 (Sox2LQY/AAA) lost the ability to repress the expression of chordin. In human neural stem cells, Affymetrix arrays revealed that 676 genes were activated and 786 genes were repressed by Sox2 overexpression. Within the genes that were repressed by Sox2, approximatly 7% were less repressed by Sox2LQY/AAA. Together, these data suggest that Sox2 functions not only as a transcriptional activator but also as a repressor through interacting with the corepressor Groucho. However, because only 7% of the repressed genes were affected by the Sox2LQY/AAA mutant, this suggests that there are other mechanisms involved in Sox2 transcriptional repressor function.

Copy number variation (CNV) has been acknowledged as an important contributor to variation in the human genome, and copy-variable regions harbouring genes are interesting subjects of research for their potential phenotypic effects. However, despite the extensive and continuing discovery of CNVs, multiallelic loci remain technically challenging to measure and study. In this thesis, a PCR-based measurement system for the tandemly repeated CNV that harbours the DEFA1A3 locus has been developed. This locus can either have the DEFA1 or the DEFA3 gene in any given copy of the repeat; the two genes differ by a single nucleotide difference in the coding sequence. This CNV has previously been shown to vary from 4 to 11 copies in a sample of the UK population. The use of this measuring system has allowed a usefully accurate measure and some characterization of this CNV in Europeans, Asians (Chinese and Japanese) and African (Ibadan, Nigeria) samples from the HapMap project, agreeing with the previously found copy number range in Europeans and showing more variability in non-Europeans. Typing of three-generation CEPH families has allowed the inference of haplotype copy numbers from segregation analysis. Combining haplotype data for some CEPH HapMap samples that were part of these families with their SNP genotypes in the same LD block has shown copy number lineages that are surprisingly well-tagged by SNPs. SNP rs4300027 allows the division of copy number haplotypes into low (2 and 3-copy) and high (4 and 5-copy) groups in European HapMap samples, a result that has been corroborated in an independent set of European samples. The Asian and African HapMap samples have failed to show this particular association. However, Japanese samples show copy number lineages tagged by other SNPs, an association not observed in other populations. In the second part of the study, an attempt has been made to explore the phenotypic effects of this variable locus. Copy number-tagging SNPs have been used to investigate published GWAS for indirect signals of association with DEFA1A3 copy number. Preliminary experiments for studying protein expression levels of DEFA1 and DEFA3 have been carried out and the possible role of this CNV as a modifier locus in Cystic Fibrosis disease severity has been investigated through direct typing of CF samples with clinical data, and indirectly through interrogating a CF GWAS.

Bacteria have evolved ways to sense and respond to changes in their population density through quorum sensing (QS) systems, and to adapt to changes in the extracellular environment through two component systems (TCS). In Pseudomonas aeruginosa, QS and the GacS/GacA TCS are global regulatory systems that modulate the expression of virulence genes at the transcriptional and post-transcriptional level, respectively. Although in P. aeruginosa the QS network has been extensively characterized, the way the Gac/Rsm global regulatory system is regulated is still unclear. The study of QS and Gac/Rsm networks is crucial for the development of new drugs able to interfere with these regulatory systems. This thesis is dedicated to the study of the Gac/Rsm global regulatory system and its interaction with the QS network. An introduction to these systems is presented in Chapter 1. The materials and methods used in this study are described in Chapter 2. In Chapter 3 the methods to detect and identify the extracellular signals modulating Gac/Rsm system are investigated. This analysis led to the identification of the Pseudomonas Quinolone Signal (PQS) molecule, which is responsible for the activation of the gene coding for the small RNA RsmZ. RsmZ (in synergy with RsmY) antagonises by titration the effects of the global post-transcriptional regulator RsmA, a small RNA-binding protein which targets specific mRNAs. Since the discovery of QS, there have been many studies showing the importance of this type of regulatory mechanism in the global transcriptional control of gene expression. However, there has been no clear evidence to attribute to QS a key role in post-transcriptional regulation of terminal gene targets. In Chapter 4, the importance of PQS in the control of lecA is demonstrated. lecA encodes for the PA-I galactophilic lectin protein whose translation rates is modulated by the activity of the regulatory small RNA rsmZ in concert with RsmA. These results demonstrate that QS not only controls terminal target gene expression at the transcriptional level, but also at the translational level. Using a genetic bank, a transposon mutagenesis and a promoter pull-down approach, new regulators were identified together with regulatory networks involved in the modulation of the global Gac/Rsm system. These results are described in Chapter 5. Chapter 6 is focused on the effect of a library of compounds available in our laboratory, for their QS-inhibiting potential. The conclusions and future directions are presented in Chapter 7.