Transcript cleavage, attenuation, and an internal promoter in the Rhodobacter capsulatus puc operon

The stoichiometry of the structural proteins of the photosynthetic apparatu s in purple photosynthetic bacteria is achieved primarily by complex regula tion of the levels of mRNA encoding the different proteins, which has been studied in the greatest detail in the puf operon. Here we investigated the transcriptional and posttranscriptional regulation of the puc operon, which encodes the peripheral light harvesting complex LHII, We show that, analog ous to the puf operon, a primary transcript encoding five pac genes is rapi dly processed to generate more stable RNA subspecies. Contrary to previous hypotheses, translational coupling and regulation of pac transcription by p ac gene products were found not to occur. A putative RNA stem-loop structur e appears to attenuate transcription initiated at the pac operon major prom oter. We also found that a minor pucD-internal promoter contributes to the levels of a message that encodes the LHII 14 kDa gamma (PucE) protein.