Microcystin is a potent inhibitor of eukaryotic protein phosphatases and has been implicated in causing hepatotoxicity to humans and animals worldwide. It is produced primarily by the bloom-forming cyanobacterium , although the function of the peptide in this micro-organism is unknown. In this study, a microcystin-related protein, MrpA, was identified using a microcystin-lacking mutant of , PCC 7806. Comparative two-dimensional protein electrophoresis showed that MrpA was strongly expressed in wild-type PCC 7806, but was not detectable in the mutant. MrpA showed similarity to the RhiA protein from , which is encoded by the operon and controlled by quorum-sensing mediators. Sequencing of flanking regions in PCC 7806 revealed the presence of a homologue, , directly downstream of . Northern blot analyses of expression in cells exposed to different light conditions revealed a rapid decline of transcription under high light conditions. Most striking was a strong increase in transcript levels from cultures irradiated with blue light. The transcription level was strongly reduced in two independent microcystin-lacking mutants under all light conditions investigated.


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