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.

Reversible changes between limiting and high inorganic carbon regimes in Synechocystis PCC6803 maintained overall protein synthesis at a high level. The protein patterns of cells during or after adaptation to these regimes were detected by pulse labelling and identification after one-dimensional and two-dimensional electrophoreses. Several particular patterns were observed. Proteins specific for either the high or the limiting carbon conditions could be identified. The protein profiles of cells having reached the stationary phase of growth clearly differed from those of their exponentially growing counterparts. Accumulation of Rubisco, the key enzyme of the Calvin cycle, was not modified in the conditions tested. z

Reversible changes between limiting and high inorganic carbon regimes in Synechocystis PCC6803 maintained overall protein synthesis at a high level. The protein patterns of cells during or after adaptation to these regimes were detected by pulse labelling and identification after one‐dimensional and two‐dimensional electrophoreses. Several particular patterns were observed. Proteins specific for either the high or the limiting carbon conditions could be identified. The protein profiles of cells having reached the stationary phase of growth clearly differed from those of their exponentially growing counterparts. Accumulation of Rubisco, the key enzyme of the Calvin cycle, was not modified in the conditions tested.