Ribosomal stalk proteins are known to play important role in protein synthesis. The ‘stalk’, an extended structure on the large subunit of the ribosome is composed mainly of two to three dimers of L12 and one L10 protein, which forms the base of the stalk. In E. coli, four copies of L12 molecules exist as dimer of dimers forming the pentameric L8 complex together with L10. This thesis is a collection of four interlinked studies on the structure, function and application of the ribosomal stalk protein L12. In the first study, we have mapped the interaction sites of the four major translation GTPase factors (IF2, EF-Tu, EF-G & RF3) on L12 molecule using heteronuclear NMR spectroscopy. Surprisingly, all these factors produced an overlapping interaction map spanning two α-helices on the C terminal domain of L12, thereby suggesting a general nature of the interaction between L12 and the GTPase factors. L12 is known to stimulate GTPase activity of the elongation factors EF-Tu and EF-G. Here, we have clarified the role of L12 in IF2 mediated initiation of protein synthesis. Our data suggest that rapid subunit association requires a specific interaction between the L12 protein on the 50S and IF2·GTP on the 30S preinitiation complex. We have also shown that L12 is not a GAP for IF2 and GTP hydrolysis triggers IF2 release from the 70S initiation complex. The next question we have addressed is why multiple copies of L12 dimer are needed on the ribosome. For this purpose, we created a pure E. coli strain JE105, where the terminal part of rplJ gene coding for the binding site of one L12 dimer on protein L10 was deleted in the chromosomal locus. Using ribosomes with single L12 dimer we have observed that the rate of the initiation and elongation involving IF2 and EF-G gets most compromised, which in turn decreases the growth rate of the bacteria.  This study also indicates that L12 can interact with different GTPase factors in a specialized manner. Lastly, we have developed an application making advantage of the multiple L12 dimers on the ribosome. By inserting a (His)6-tag at the C-terminus of the L12 protein we have created a novel E. coli strain (JE28), where all ribosomes are tetra-(His)6-tagged. Further, we have developed a single step method for purification of the active (His)6-tagged ribosomes from JE28.

We are interested in complex electronic structures of various atomic and ionics systems. We use an ab initioapproach, the multiconfigurational Dirac-Hartree-Fock (MCDHF), to compute atomic structures and properties.We contribute in three main ways to the already existent literature: by developing and implementing originalcomputer programs, by investigating possibilities of alternative computational methodologies and strategies, andfinally by performing accurate atomic structure calculations to support other research fields, i.e. nuclear physics,astrophysics or experimental physics, through the theoretical estimation of relevant atomic data.We raise questions about the choice of the optimal orbital basis by considering finite basis sets, MCDHF orbitalbases and natural-orbital bases. We demonstrate the promising potential of the latter in the context of hyperfinestructures and hope that others will find interest in pursuing our analysis. Ultimately, our work put forward someweaknesses of the traditional optimization strategy based on the layer-by-layer optimization strategy.We also perform large-scale calculations to determine accurate atomic properties such as energy levels, hyperfinestructures, isotope shifts, transition parameters, radiative lifetimes and Landé g factors. We show through thevariety of atomic properties and atomic systems studied, the difficulty of describing, in the relativistic framework,the correlation between the spatial position of electrons due to their Coulomb repulsion.This thesis is organized in two main parts. The first one is dedicated to the theoretical and computationalbackgrounds that are needed to understand the theoretical models and the interpretation of our results. Thesecond part presents and summarizes our articles and manuscripts. They are separated in four groups, A, B, C,and D, around the themes of the atomic orbital bases, the applications to nuclear physics, the applications toastrophysics, and investigations of negative ions. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished