CLONING AND CHARACTERIZATION OF THE CHLOROPHYLL BIOSYNTHESIS GENE CHLM FROM SYNECHOCYSTIS PCC-6803 BY COMPLEMENTATION OF A BACTERIOCHLOROPHYLL BIOSYNTHESIS MUTANT OF RHODOBACTER-CAPSULATUS

A bacteriochlorophyll a biosynthesis mutant of the purple photosynthet ic bacterium Rhodobacter capsulatus was functionally complemented with a cosmid genomic library from Synechocystis sp. PCC 6803. The complem ented R. capsulatus strain contains a defined mutation in the bchM gen e that codes for Mg-protoporphyrin IX methyltransferase, the enzyme wh ich converts Mg-protoporphyrin IX to Mg-protoporphyrin IX methylester using S-adenosyl-L-methionine as a cofactor. Since chlorophyll biosynt hesis also requires the same methylation reaction, the Synechocystis g enome should similarly code for a Mg-protoporphyrin IX methyltransfera se. Sequence analysis of the complementing Synechocystis cosmid indica tes that it contains an open reading frame exhibiting 29 % sequence id entity to BchM. In addition, expression of the Synechocystis gene in t he R. capsulatus bchM mutant via the strong R. capsulatus puc promoter was shown to support nearly wild-type levels of bacteriochlorophyll a synthesis. To our knowledge, the Synechocystis sequence thus represen ts the first chlorophyll biosynthesis gene homolog of bchM. The comple menting Synechocystis cosmid was also shown to code for a gene product that. is a member of a highly conserved family of RNA binding protein s, the function of which in cyanobacteria remains undetermined.