Studies on mRNA turnover in bacteria

Sammanfattning: Genetic information, which is stored in DNA, is transferred to messenger RNA (mRNA) in a process called transcription. mRNAs are then translated by ribosomes into proteins which, with some important exceptions, catalyze chemical reactions in cells. Since cells must be able to adapt to changes in the environment, mRNAs must be unstable. The present investigation concerns mRNA degradation mainly in Bacillus subtilis, which is the model organism for Gram-positive bacteria. Comparative studies have also been performed in the Gram-negative Escherichia coli which is the best characterized bacterium concerning mRNA degradation. I have studied expression of the glpD gene, encoding glycerol-3-phosphate dehydrogenase, which is necessary for growth of B. subtilis on glycerol as the sole carbon and energy source. Previous studies have shown that expression of this gene is controlled by a protein, GlpP, which functions both as an antiterminator of transcription and an mRNA stabilizing protein in B. subtilis. The present investigation shows that GlpP binds to glpD mRNA both in vivo and in vitro. In E. coli, GlpP functions as an antiterminator protein but it does not stabilize glpD mRNA. The endoribonucleolytic cleavage pattern of glpD mRNA also differs between the two organisms and the major, RNase III-dependent, processing site found in E. coli is hardly detectable in B. subtilis. Furthermore, the secondary structure of the untranslated leader region of glpD mRNA has been determined both in vivo and in vitro. Based on these findings, a model for GlpP-mediated antitermination and mRNA stabilization is proposed. Studies have also been performed on the expression of the B. subtilis aprE gene, which encodes the excreted protease subtilisin. The results show that the extreme aprE mRNA stability is determined by the 5' region and is independent of growth phase.