TRANSPOSON INSERTION IN GENES-CODING FOR THE BIOSYNTHESIS OF STRUCTURAL COMPONENTS OF THE ANABAENA SP. PHYCOBILISOME

Anabaena sp. PCC 7120 mutants defective in phycobiliprotein biosynthes is or phycobilisome assembly were generated by transposon mutagenesis. Four mutants with grossly reduced content of the major phycobiliprote in, phycocyanin, were found to have insertions within the cpcBACDEFG1G 2G3G4 operon coding for phycocyanin biosynthesis and assembly. The ins ertion in mutant B646 separated the promoter from the open reading fra mes and eliminated production of the phycocyanin alpha (CpcA) and beta (CpcB) subunits. Insertion in cpcC in mutant B642 eliminated producti on of the L-R(36) linker polypeptide required for assembly of phycocya nin into the distal discs of the phycobilisome rod substructures. Muta nts B64328 and B64407 had insertions, respectively, in cpcE and cpcF, genes coding for the subunits of the heterodimeric lyase which catalyz es the attachment of phycocyanobilin to the phycocyanin apo-alpha subu nit. Mutant SB12, often unable to survive under low light, was found t o have an insertion in the apcE gene coding for the large core-membran e linker (L-CM(128)) that provides the scaffold for assembly of the ph ycobilisome core. DNA sequencing 3' of apcE revealed genes apcABC, cod ing for the alpha and beta subunits of allophycocyanin and for the sma ll core linker L-C(7.8). Amino acid sequence comparisons showed that t he ApcA and ApcB proteins are 37% identical and that each of these pol ypeptides is highly similar to corresponding polypeptides from the dis tantly related filamentous strains Calothrix sp. PCC7601 and Mastigocl adus laminosus.