CHARACTERIZATION OF A LIGHT-RESPONDING TRANSACTIVATOR RESPONSIBLE FORDIFFERENTIALLY CONTROLLING REACTION-CENTER AND LIGHT-HARVESTING-I GENE-EXPRESSION IN RHODOBACTER-CAPSULATUS

The purple nonsulfur photosynthetic bacterium Rhodobacter capsulatus r egulates synthesis of its photosystem in response to two environmental stimuli, oxygen tension and light intensity. Here we describe the ide ntification and characterization of the trans-acting regulatory gene h vrA, which we show is involved in differentially controlling reaction center and light-harvesting gene expression in response to alterations in light intensity. An hvrA mutant strain is shown to lack the capabi lity to trans-activate light-harvesting-I and reaction center gene exp ression but retain normal light-harvesting-II and photopigment regulat ion, in response to a reduction in light intensity. As a consequence o f altered expression, hvrA mutant strains exhibit reduced photosynthet ic growth capabilities under dim-light conditions. The results of this study acid additional studies indicate that regulated synthesis of th e photosystem involves complex sets of overlapping regulatory circuits that differentially control photosystem gene expression in response t o environmental stimuli such as oxygen tension and light intensity.