Branched-chain Amino Acid Biosynthesis in the Blue-green Alga Anabaena variabilis

Abstract

SUMMARYFour enzymes (threonine deaminase, a-acetohydroxyacid synthetase, isopropylmalate synthetase and t ransaminase B) involved in leucine, isoleucine and valine synthesis have been detected in extracts of the blue-green alga Anabaena variabilis and the control of three of the enzymes examined. A new procedure for the determination of transaminase B is described. Threonine deaminase and a-acetohydroxyacid synthetase are subject to feedback control by their end-products. No evidence for the repression of enzyme synthesis in the pathway has been found, although the assimilation and incorporation of exogenous valine and leucine was demonstrated.

Regulation by external pH and stationary growth phase of the acetolactate synthase from Synechocystis PCC6803

“…No regulating effect of valine on growth was observed in the WT or the M4 mutant of Synechocystis PCC6803, in agreement with observations on A. nidulans and A. variabilis (Hood and Carr, 1972) and in contrast to those on S. platensis (Riccardi et al ., 1988). This result confirms the observation by Labarre et al .…”

“…In the latter strain, the two corresponding enzymes display optimal pHs of 6 and 7.5 respectively (Forlani et al ., 1991). Two optimal pHs for acetolactate synthase activity have also been observed in extracts of A. variabilis (Hood and Carr, 1972). In Synechocystis PCC6803, genes have been identified for an acetohydroxy acid isomeroreductase (Fig.…”

“…The situation is variable among cyanobacteria. Biosynthesis of valine/leucine and isoleucine by the established pathways has been demonstrated by detection of several of the activities concerned, including acetolactate synthesis in Anabaena variabilis and Anacystis nidulans (renamed Synechococcus PCC6301) (Hood and Carr, 1972). Complementation of an AHAS‐deficient E. coli triple mutant has been achieved by expression of a gene homologous to the E. coli ilv genes from Synechococcus PCC7942 (Friedberg and Seijffers, 1990) and of at least one of the two homologous genes present in Spirulina platensis (Forlani et al ., 1991; Riccardi et al ., 1991; Milano et al ., 1992).…”

The Control of Homoserine-O-transsuccinylase in a Methionine-requiring Mutant of the Blue-green Alga Anacystis nidulans

“…For example, the inclusion in the growth medium of valine or isoleucine, singly or in combination, did not alter the activities of threonine deaminase, a-acetohydroxyacid synthetase or transaminase B in A . variabilis (Hood & Carr, 1972). End-product interaction with existing enzymes was, however, readily shown.…”