Transposon insertion in genes coding for the biosynthesis of structural components of the Anabaena sp. phycobilisome

Abstract

Anabaena sp. PCC 7120 mutants defective in phycobiliprotein biosynthesis or phycobilisome assembly were generated by transposon mutagenesis. Four mutants with grossly reduced content of the major phycobiliprotein, phycocyanin, were found to have insertions within the cpcBACDEFG1G2G3G4 operon coding for phycocyanin biosynthesis and assembly. The insertion in mutant B646 separated the promoter from the open reading frames and eliminated production of the phycocyanin α (CpcA) and β (CpcB) subunits. Insertion in cpcC in mutant B642 eliminated production of the L36Rlinker polypeptide required for assembly of phycocyanin into the distal discs of the phycobilisome rod substructures. Mutants B64328 and B64407 had insertions, respectively, in cpcE and cpcF, genes coding for the subunits of the heterodimeric lyase which catalyzes the attachment of phycocyanobilin to the phycocyanin apo-α subunit. Mutant SB12, often unable to survive under low light, was found to have an insertion in the apcE gene coding for the large core-membrane linker (L128CM) that provides the scaffold for assembly of the phycobilisome core. DNA sequencing 3′ of apcE revealed genes apcABC, coding for the α and β subunits of allophycocyanin and for the small core linker L7.8C. Amino acid sequence comparisons showed that the ApcA and ApcB proteins are 37% identical and that each of these polypeptides is highly similar to corresponding polypeptides from the distantly related filamentous strains Calothrix sp. PCC7601 and Mastigocladus laminosus.