THE SUPEROPERONAL ORGANIZATION OF GENES FOR PIGMENT BIOSYNTHESIS AND REACTION CENTER PROTEINS IS A CONSERVED FEATURE IN RHODOBACTER-CAPSULATUS - ANALYSIS OF OVERLAPPING BCHB AND PUHA TRANSCRIPTS

Most of the essential biosynthetic and structural genes involved in bacterial photosynthesis are clustered in a 46 kb region of the Rhodobacter capsulatus genome. Previous analyses have demonstrated that the puf operon, which encodes light harvesting and reaction center structural genes as well as a regulatory gene for bacteriochlorophyll biosynthesis, is expressed from a complex set of overlapping transcripts. Differential initiation and processing of these transcripts is thought to be involved in regulating expression of puf-encoded genes. In this study we demonstrate that the puh operon, which is located 39 kb away from the puf operon, also contains overlapping transcripts. One large 11 kb puhA transcript is shown to be a product of read-through from an upstream operon (bchB) which encodes numerous bacteriochlorophyll biosynthesis genes. A second 1.1 kb mRNA is shown to be derived from the 11 kb bchB transcript by processing and a third, highly expressed, 0.95 kb transcript is shown to be initiated from a promoter located within the distal gene of the bchB operon. The occurrence of overlapping transcripts for the puf and puh operons was further shown to influence development of the photochemical apparatus during conditions of environmental shifts in oxygen tension. Evidence for the occurrence of a " superoperonal " organization of overlapping operons in several different species of purple photosynthetic bacteria is discussed.