Transcription of three sets of genes coding for the core light-harvesting proteins in the purple sulfur bacterium, Allochromatium vinosum.

Abstract

The nucleotide sequence of the puf operon coding for the subunits of the photosynthetic reaction center and the core light-harvesting complex (LH1) of the purple sulfur bacterium, Allochromatium (A.) vinosum (formally Chromatium vinosum), was completely determined. Unlike other known puf operons, which contain only one set of genes coding for the LH1 apoproteins, pufB and pufA, the A. vinosum puf operon included three sets of pufB and pufA genes with a gene order of pufB (1) A (1) LMCB (2) A (2) B (3) A (3). Northern hybridization analysis suggested that all of the nine puf genes are co-transcribed as a 4.43 kb mRNA. Three small mRNAs corresponding to pufB (2) A (2) B (3) A (3), pufB (2) A (2) B (3), and pufB (2) A (2) were detected, as well as two small mRNAs covering pufB (1) A (1). Analysis of the nucleotide sequence of the puf operon, including the flanking regions and 5'-ends of the six mRNAs, suggested that the transcription of the A. vinosum puf operon is initiated at 74 bp downstream from the bchZstop codon (295 bp upstream from the pufB (1) start codon), and regulated by a promoter located at its direct upstream. The possible promoter is overlapped with a binding motif of a repressor protein for pigment-biosynthesis genes, PpsR or CrtJ, known in other purple bacteria. No other possible promoters were found within the puf genes. These findings indicate that three sets of pufA and pufB genes of A. vinosum are co-transcribed as a long mRNA containing all the puf genes, and, from this long mRNA, the five short mRNAs are possibly derived by post-transcriptional modifications.