Control of gene expression in blue-green algae

THE blue-green algae are a major group of prokaryotes in many ways resembling, and perhaps phylogenetically related to, the chloroplasts of photosynthetic eukaryotes. Metabolic control mechanisms in this group are of interest because of its unique evolutionary position and because many of its members are obligate photoautotrophs and as such may show patterns of control different from those known in heterotrophic bacteria. Indeed, Carr and his collaborators_1-3 have proposed that blue-greens do not in general regulate metabolism at the level of gene expression, as heterotrophs commonly do. An exogenous metabolite may be assimilated, but its presence in the cell does not induce (or repress) the synthesis of enzymes responsible for its own catabolism (or biosynthesis). Thus, these workers argue, exogenous substrates do not usually support significant dark growth, or even stimulate growth in the light, because blue-greens simply cannot adjust the levels of the enzymes of intermediary metabolism to accommodate any source of carbon other than CO ₂ or any source of energy other than light¹.