Regulation by external pH and stationary growth phase of the acetolactate synthase from Synechocystis PCC6803.
Several characteristics identify the protein encoded by the alsS gene [sll1981 in Cyanobase (http://www.kazusa.or.jp/cyano/cyano. html)] of Synechocystis PCC6803 as an acetolactate synthase. The AlsS protein is about 60% homologous to the AlsS from Bacillus subtilis or other bacteria. These enzymes condense two pyruvates to form acetolactate, implicated in pH homeostasis via the acetoin-2, 3-butanediol pathway or in valine biosynthesis. Transcriptional fusions revealed that alsS was induced at the onset of stationary phase, as in B. subtilis, a situation leading to an increase in the pHout to above 11 in Synechocystis. This is the first cyanobacterial gene showing a dependence on pH for its expression. Induction was also obtained by the presence of > 100 mM Na+, the effect being prevented by amiloride, in agreement with Na+/H+ exchange in the pH homeostasis process. Homology of the Synechocystis AlsS protein to the close family of acetohydroxy acid synthases (including one in Synechocystis) is around 30%. These enzymes are involved in the parallel routes for valine/leucine and isoleucine biosynthesis. No phenotype of auxotrophy for any of these amino acids was associated with a null mutation in the Synechocystis alsS gene. The AlsS enzyme did not complement the isoleucine deficiency of an acetohydroxy acid synthase-deficient Escherichia coli mutant.
Maestri
O
O
Laboratoire de Chimie Bactérienne, CNRS, Marseilles, France.
Joset
F
F
D000095:Acetolactate Synthase / Q000235:genetics / Q000378:metabolism*; D019677:Artificial Gene Fusion; D003001:Cloning, Molecular; D000458:Cyanobacteria / Q000201:enzymology* / Q000235:genetics / Q000254:growth & development; D004926:Escherichia coli / Q000235:genetics; D015971:Gene Expression Regulation, Enzymologic; D005816:Genetic Complementation Test; D006863:Hydrogen-Ion Concentration; D014158:Transcription, Genetic
D000095:Acetolactate Synthase