Organization and transcription of the genes encoding two differentially expressed phycocyanins in the cyanobacterium Pseudanabaena sp. PCC 7409

The cpc1 and cpc2 operons of the group III chromatically adapting cyanobacterium Pseudanabaena sp. PCC 7409 were isolated and their nucleotide sequences determined. The cpc1 operon consists of the genes cpcB1A1EF and gives rise to an abundant 1400-nucleotide transcript encoding the cpcB1A1 genes and two low-abundance transcripts of 1000 nucleotides and 1100 nucleotides encoding the cpcF gene. Two extremely low-abundance transcripts of approximately 2900 nucleotides and 4800 nucleotides possibly encode cpcB1A1E and cpcB1A1EF, respectively. All transcripts were present in cultures grown in either red or green light. The transcription start of the cpcB1A1 mRNA was mapped to a position 238 bp 5′ to the cpcB1 translation start. The cloned fragment containing the cpcB2A2 genes was found to contain only a portion of the cpc2 operon and consisted of the cpcB2A2 genes and the 5′ portion of the linker gene cpcH2. On the basis of biochemical evidence, as well as sequence data from other cpc operons, it is probable that the complete Pseudanabaena sp. PCC 7409 cpc2 operon consists of the genes cpcB2A2H2I2D2. This operon is almost exclusively transcribed in cells grown in red light and gives rise to an abundant mRNA 1400 nucleotides in length that encodes the cpcB2A2 genes. A second transcript of 2400 nucleotides encodes the cpcB2A2H2 genes. A third transcript of 3800 nucleotides encodes the cpcB2A2H2 genes and probably the cpcI2 and cpcD2 genes as well. Transcription of the cpc2 mRNAs inititates 219 bp 5′ to the cpcB2 translation start. The promoter region of the Pseudanabaena sp. PCC 7409 cpc1 operon contains the sequence 5′ ttGTATaa 3′ that is also found to occur within 20 bp of the transcription initiation sites of a number of other constitutively expressed cpc promoters. A high level of sequence similarity also occurs between the red-light-inducible cpc2 promoters of Pseudanabaena sp. PCC 7409 and Calothrix sp. PCC 7601.