MANGANOUS IONS SUPPRESS PHOTOSYNTHESIS GENE-EXPRESSION IN RHODOBACTER-SPHAEROIDES

The effect of manganous ions [Mn(II)] and ferrous ions [Fe(II)] on exp ression of photosynthesis genes in Rhodobacter sphaeroides was investi gated. The presence of Mn(II) during phototrophic (anaerobic) and chem otrophic (aerobic) growth of R. sphaeroides caused a decrease in the a mount of bacteriochlorophyll and carotenoid pigments which were synthe sized and this was associated mainly with a decrease in the level of l ight-harvesting complex II. Mn(II) was shown to cause a decrease in ex pression of the puc operon, which encodes the polypeptides of light-ha rvesting complex II. Expression of the puc operon is controlled by the central repressor of photosynthesis gene expression, PpsR. In a ppsR mutant there was no effect of Mn(II) on photosynthesis gene expression . It is concluded that Mn(II) may act as a corepressor in the action o f PpsR or act via an as yet uncharacterized protein that interacts wit h PpsR. In contrast to the effects of Mn(II), Fe(II) was required for high levels of photosynthesis gene expression. This requirement for Fe (II) was shown to be related to the regulation of hemA, a gene under t he control of the transcriptional regulator, FnrL. Mn(II) did not affe ct FnrL-dependent gene expression.