Date of Completion

January 1984


Biology, Microbiology




Even though L-alanine is an important primary metabolite in bacteria, little is known about its biosynthesis in any microorganism. This thesis begins the study of the biosynthesis of L-alanine in Escherichia coli and Salmonella typhimurium.^ Mutations affecting L-alanine biosynthesis were obtained by isolating valine-requiring derivatives of L-isoleucine-requiring ilvE mutants, which lack the branched-chain amino acid transaminase, transaminase B. Two classes of transposon-induced mutants were isolated in E. coli and S. typhimurium: one class of mutants (avtA) lacked TrC and had a L-valine requirement which could not be satisfied by any other amino acid, and a second class (ala) retained TrC activity and could grow on L-alanine in place of L-valine.^ Neither avtA or ala mutations alone confer a growth requirement in E. coli or S. typhimurium. It is only when they are combined with other mutations that avtA- and ala-dependent growth requirements are detected. avtA mutations confer: upon ilvE mutants an L-valine requirement, upon ilvGEDA mutants of E. coli an L-leucine and pantothenate requirement, upon ilvA mutants a reduced ability to use (alpha)-aminobutyrate in place L-isoleucine, and upon ala mutants a partial L-alanine requirement. ala mutations confer: upon ilvE mutants an L-alanine or L-valine requirement, and upon avtA mutants a partial L-alanine requirement.^ The ala mutations were shown to reduce alanine-glutamate transaminase activity (ALAGT) in E. coli and S. typhimurium, suggesting that this enzyme is involved in L-alanine biosynthesis. However, no mutations abolished this activity in either genus, leaving assignment of its role in L-alanine synthesis tentative. These observation suggest the L-alanine is synthesized by at least two pathways: by TrC and by ALAGT.^ The regulation of synthesis of these enzymes suggests that they are regulated by two distinct regulatory pathways: TrC synthesis was repressed by L-alanine, L-leucine, and a number of nonprotein amino acids whereas ALAGT synthesis was unaffected. Also, ALAGT synthesis increased as alanyl-tRNA charging decreased whereas TrC synthesis was unaffected. The control of the levels of TrC and ALAGT by L-alanine and alanyl-tRNA suggests that these enzymes are primarily L-alanine biosynthetic enzymes. ^