Genetic analysis of phycobilisome mutants in the cyanobacterium Synechococcus species PCC 6301

The chromophoric protein phycocyanin is the major protein in the phycobilisome rod of the cyanobacterium Synechococcus 6301 (formerly designated Anacystis nidulans sp. UTEX 625). The gene clusters coding for the beta- and alpha-subunits of phycocyanin are duplicated on the chromosome of Synechococcus 6301 and separated by an intergenic region 2.5 kb long. The structure of the phycocyanin operons of the phycobilisome mutant strains AN112 and AN135 was compared to that of wild-type Synechococcus 6301. Both mutants have an altered phycobilisome structure resulting in the appearance of rods of a shorter overall length. Genetic mapping indicated that the mutant strain AN112 completely lacked the intergenic region and carried only one set of phycocyanin subunit genes. No gross structural difference in the genetic organization of the corresponding region of mutant strain AN135 was detected. Northern blot analysis and primer extension experiments were used to monitor the transcriptional pattern of the phycocyanin rod operon. It was found that AN112 had lost the 3.7kb minor mRNA, which covers the intergenic region, and only produced two major 1.3 and 1.4kb mRNA species. These transcripts were identified as fusion products of the 5' end of the transcriptional unit originating from the promoter region of the left-hand phycocyanin gene cluster and the 3' end of the transcriptional unit covering the right-hand phycocyanin gene cluster. The mutant strain AN135 had a transcriptional pattern very similar to that of the wild type. The level of transcription of the major transcripts covering the phycocyanin gene clusters was similar in the wild-type and mutant strains.(ABSTRACT TRUNCATED AT 250 WORDS)