Pigment biosynthesis in the cyanobacterium, Anacystis nidulans, was examined in the presence of gabaculine (5-amino-1,3-cyclohexadienylcarboxylic acid). At 20 micromolar, this inhibitor blocked the biosynthesis of both chlorophyll and phycocyanin. Analogs of gabaculine were not effective as inhibitors of chlorophyll or phycocyanin biosynthesis. Iron-and phosphate-deficient cultures were 2-to 4-fold more sensitive to the inhibitor than were normal or nitrate-deficient cultures. Inhibition resulted in the excretion of a mixture of organic acids by the cells. 5-Aminolevulinic acid was a principle component of the mixture, identified by thin layer chromatography. Excretion of 6-aminolevulinic acid occurred following a brief lag after gabaculine addition. It remained linear for nearly 24 hours and was dependent upon illumination. However, high light inhibited excretion. Apparently, gabaculine blocks chlorophyll biosynthesis after the formation of 6-aminolevulinic acid in cyanobacteria.Recovery of cyanobacteria from iron deficiency is accompanied by a reproducible pattern of photosynthetic membrane repair. When iron is restored to a chlorotic culture, there is a 3 to 5 h lag, followed by the synthesis and insertion of Chl proteins into existing membranes (23). At later times, the assembly of new membrane becomes apparent (27). The factors which regulate the assembly of Chl proteins under these conditions are not well understood. Iron starvation reduces the synthesis of protoporphyrins, presumably by feedback inhibition due to an accumulation of protoheme (5,8,28). Since Chl is a component of several thylakoid membrane polypeptides, decreased Chl availability may play a regulatory role in Chl protein assembly.The differential effects of Chl availability and of intracellular iron during recovery from iron deficiency have previously been examined using two inhibitors of Chl biosynthesis (12). One of these, LA2, is a competitive inhibitor of ALA dehydratase (17,18), and effectively blocks protoporphyrin biosynthesis in both normal and iron-deficient cyanobacteria (1 1). The mechanism by which the second inhibitor, GAB, affects cyanobacteria is virtually unknown (12). This compound interferes with pyridoxal phosphate-linked aminotransferase activity (25,26), and inhibits Chl biosynthesis in higher plants (9,10,15,29).In this report, we have examined the effects of GAB on Our findings consist of three pertinent observations. First, GAB inhibited the accumulation of both Chl and PC. The concentration requirements for inhibition were similar to those of other systems (15,25). Second, these concentration requirements were influenced by the nutrient status of the cell. Finally, GAB induced the excretion of organic acids, a major excretion product being ALA. This suggested that GAB inhibits Chl biosynthesis in cyanobacteria at a site following ALA formation. MATERIALS AND METHODSCells of Anacystis nidulans R2 were grown in shaking culture as previously described (13). Axenic cultures were monitored microscopically and by...