In this thesis we investigate the importance of various elements of the input physics and other parameters which affect the structure and evolution of models of low mass stars. Of the elements of input physics, the nuclear generation rates, the electron screening effect on thermo-nuclear reactions, and the conductive opacities are adopted from the formulation or data tables by other contributors. The low temperature opacities are taken from the data calculated by Carson and Sharp, as well as those by Alexander. In our study, we establish a sophisticated model for the determination of the equation of state. It is formulated by a new method based on the theory of the grand canonical ensemble. The interatomic interactions, which are responsible for the nonideal effects and pressure ionization, are treated carefully in the equation of state. In addition, and consistent with the equation of state, we also evaluate the radiative opacity according to the average atom model for heavy elements and the hydrogen-like model for hydrogen and helium. Negative hydrogen absorption and free electron scattering are also included. A computing code is constructed to calculate the radiative opacity data which are required in the study of low mass stars. To investigate the importance of each element of the input physics, we exclude it or replace it with an alternative for the model calculations. The parameters of the stellar code, such as the element abundances, the surface condition and the mixing length ratio are as well investigated by alternative values or formulation. In the computation, all elements of input physics, except the energy generation rates, are incorporated into the main code by means of data tables. A bicubic interpolation is used for their input. Our numerical calculations cover the zero age main sequence of the stars ranging from a solar mass down to the hydrogen burning minimum mass. The calculated results of the zero age models for the Standard Population I and Population II indicate good agreement with the observed data for the objects with effective temperatures above 3000 K, or with masses greater than 0,15 M[sub]sun. Below this limit, in agreement with other theoretical work, there is still a small discrepancy between the theoretical models and the observed data. The investigations of the effects of the input physics and model parameters show that some of them affect considerably the minimum mass limit for hydrogen burning on the lower main sequence. The low temperature opacities, the nonideal effects in the EOS and the conductive opacities lead to a limiting mass ranging from 0.08 M[sub]sun to 0.15 M[sub]sun although they do not affect the models with masses above 0.15 M[sub]sun obviously. The investigations also show that the massive models of the lower main sequence (i.e. those with mass near the sun) are dependent very much on the element abundances and the mixing length ratio while the lower mass models are not. The lower main sequence models are found to be insensitive to the surface condition (photospheric model or atmospheric model) used. We also perform the calculation of the evolutionary models of the lower main sequence from the zero age up to an age of 1010 years. A perfect theoretical model of the solar evolution is obtained when the atmospheric model is used as surface condition, the element abundances are chosen to be 0.70/0.28/0.02 and the mixing length ratio to be 1.5. The investigations show that the element abundances are the most important parameters in the determination of the solar models. The mixing length ratio is the second most important one being much more important than the physical model used as surface condition. The evolutionary models of the masses below 0.8 M[sub]sun are found to have only small changes in their properties within 1010 years. None of them can deplete their central hydrogen in that time. We find one problem in that the models with masses around 0.1 M[sub]sun have oscillating solutions for both the zero age models and evolutionary models. According to our investigation, the nonideal effects in the EOS can be responsible. These oscillating solutions imply the existence of more than one stable configuration for stellar masses.

The process of decoding is the most crucial determinant of the quality of protein synthesis. Ribosomal protein L9 was first implicated in decoding fidelity when a mutant version of L9 was found to increase the translation of a T4 phage gene. Later studies confirmed that the absence of L9 leads to increased translational bypassing, frameshifting, and stop codon readthrough. L9 is part of the large subunit of the prokaryotic ribosome and is located more than 90 Å from the site of decoding, making it difficult to envision how it might affect decoding and reading frame maintenance. Twenty years after the identification of L9's putative function, there is no mechanism for how a remotely located L9 improves translation fidelity. This mystery makes our picture of translation incomplete. Despite the high conservation of L9 in eubacteria, E.coli lacking L9 does not exhibit any obvious growth defects. Thus, the evolutionary advantage conferred by L9 in bacteria is masked under laboratory conditions. In order to uncover unique L9-dependent conditions, a library of E. coli mutants was screened to isolate those that rely on L9 for fitness. Interestingly, factors found to be synergistic with L9 had no known role in fidelity. Six independent mutants were isolated, each exhibiting a severe growth defect that is partially suppressed in the presence of L9. One class of L9-dependent mutations was present in an essential ribosome biogenesis factor, Der. Der's established function is in the maturation of the large ribosomal subunit. The identified mutations severely impaired the GTPase activity of Der. Interestingly, L9 did not directly compensate for the defective GTPase activity of mutant Der. The second class of L9-dependent mutations was present in EpmA and EpmB, factors required to post-translationally modify elongation factor, EF-P. EF-P's established function is in the translation of poly-proline containing proteins. EF-P deficient cells were nearly inviable in the absence of L9; however, L9 did not directly influence poly-proline translation. Therefore, in each case, L9 improved cell health without altering the activity of either Der or EF-P. Remarkably, the der mutants required only the N domain of L9, whereas the absence of active EF-P required full-length, wild-type L9 for growth complementation. Thus, each mutant class needed a different aspect of L9's unique architecture. In cells lacking either active EF-P or Der, there was a severe deficiency of 70S ribosomes and the indication of small subunit maturation defects, both of which worsened upon L9 depletion. These results strongly suggest that L9 plays a role in improving ribosome quality and abundance under certain conditions. Overall, the genetic screen lead to the discovery that bacteria need L9 when either of two important translation factors (Der or EF-P) is inactivated. This work has characterized the physiological requirement for L9 in each case and offers a new insight into L9's assigned role in translation fidelity.

Applications of natural gases that contain high levels of hydrogen have become a primary interest in the gas turbine market. For reheat gas turbines, understanding of the ignition delay times of high-hydrogen natural gases is important for two reasons. First, if the ignition delay time is too short, autoignition can occur in the mixer before the primary combustor. Second, the flame in the secondary burner is stabilized by the ignition delay time of the fuel. While the ignition delay times of hydrogen and of the individual hydrocarbons in natural gases can be considered well known, there have been few previous experimental studies into the effects of different levels of hydrogen on the ignition delay times of natural gases at gas turbine conditions. In order to examine the effects of hydrogen content at gas turbine conditions, shock-tube experiments were performed on nine combinations of an L9 matrix. The L9 matrix was developed by varying four factors: natural gas higher-order hydrocarbon content of 0, 18.75, or 37.5%; hydrogen content of the total fuel mixture of 30, 60, or 80%; equivalence ratios of 0.3, 0.5, or 1; and pressures of 1, 10, or 30 atm. Temperatures ranged from 1092 K to 1722 K, and all mixtures were diluted in 90% Ar. Correlations for each combination were developed from the ignition delay times and, using these correlations, a factor sensitivity analysis was performed. It was found that hydrogen played the most significant role in ignition delay time. Pressure was almost as important as hydrogen content, especially as temperature increased. Equivalence ratio was slightly more important than hydrocarbon content of the natural gas, but both were less important than pressure or hydrogen content. Further analysis was performed using ignition delay time calculations for the full matrix of combinations (27 combinations for each natural gas) using a detailed chemical kinetics mechanism. Using these calculations, separate L9 matrices were developed for each natural gas. Correlations from the full matrix and the L9 matrix for each natural gas were found to be almost identical in each case, verifying that a thoughtfully prepared L9 matrix can indeed capture the major effects of an extended matrix.

Studies of ribosomal reading frame maintenance are often based on frameshift mutation suppression experiments. In this thesis, suppression of a frameshift mutation in Salmonella enterica serovar Typhimurium by a tRNA and a ribosomal protein are described. The +1 frameshift mutation hisC3072 (that contains an extra G in a run of Gs) is corrected by mutations in the argU gene coding for the minor tRNAArgmnm5UCU. The altered tRNAArgmnm5UCU has a decreased stability and reduced aminoacylation due to changed secondary and/or tertiary structure. Protein sequencing revealed that during the translation of the GAA-AGA frameshifting site the altered tRNAArgmnm5UCU reads the AGA codon inefficiently. This induces a ribosomal pause, allowing the tRNAGlumnm5s2UUC residing in the ribosomal P-site to slip forward one nucleotide. The same frameshift mutation (hisC3072) was also suppressed by defects in the large ribosomal subunit protein L9. Single base substitutions, truncations, and absence of this protein induced ribosome slippage. Mutated ribosome could shift to the overlapping codon in the +1 frame, or bypass to a codon further downstream in the +1 frame. The signal for stimulation of slippage and function of L9 needs to be investigated. During the search for suppressors of the hisD3749 frameshift mutation, a spontaneous mutant was isolated in the iscU gene that contained greatly decreased levels of the thiolated tRNA modifications ms2io6A and s2C. The iscU gene belongs to the iscR-iscSUA-hscBA-fdx operon coding for proteins involved in the assembly of [Fe-S] clusters. As has been shown earlier, IscS influences the synthesis of all thiolated nucleosides in tRNA by mobilizing sulfur from cysteine. In this thesis, it is demonstrated that IscU, HscA, and Fdx proteins are required for the synthesis of the tRNA modifications ms2io6A and s2C but are dispensable for the synthesis of s4U and (c)mnm5s2U. Based on these results it is proposed that two distinct pathways exist in the formation of thiolated nucleosides in tRNA: one is an [Fe-S] cluster-dependent pathway for the synthesis of ms2io6A and s2C and the other is an [Fe-S] cluster-independent pathway for the synthesis of s4U and (c)mnm5s2U. MiaB is a [Fe-S] protein required for the introduction of sulfur in ms2io6A. TtcA is proposed to be involved in the synthesis of s2C. This protein contains a CXXC conserved motif essential for cytidine thiolation that, together with an additional CXXC motif in the C-terminus may serve as an [Fe-S] cluster ligation site.