Linker polypeptides of the phycobilisome from the cyanobacterium Mastigocladus laminosus: amino-acid sequences and relationships

Abstract

Three linker polypeptides of the phycobilisome from the cyanobacterium Mastigocladus laminosus were isolated: A 8.9-kDa polypeptide, L8.9R(C), which is probably associated with C-phycocyanin, a 34.5-kDa polypeptide, L34.5,PCR, which forms a complex with C-phycocyanin, and a 34.5-kDa polypeptide, L34.5,PECR, which is linked to phycoerythrocyanin. The complete amino-acid sequence (80 residues) of the L8.9R(C) polypeptide was determined as well as the N-terminal 44 residues of both L34.5R polypeptides and the 114 C-terminal residues of L34.5,PECR. L8.9R(C) is homologous to L8.9C (Füglistaller et al. (1984) Hoppe-Seyler's Z. Physiol. Chem. 365, 1085-1096) and to the C-terminal sequence of L34.5,PECR. The N-terminal sequences of L34.5,PECR and L34.5,PCR exhibit 34% homology. The 44 N-terminal residues of L34.5,PECR are related to the beta-subunit of phycoerythrocyanin (23% homology), while the C-terminal sequence of L34.5,PECR is more related to alpha PEC (21% homology within 60 residues). This suggests that the 30-kDa-linker polypeptide family originates from a fusion of the alpha- and beta-subunit genes and the corresponding intercistronic DNA sequence, as might have arisen through mutation in the stop-codon of the beta-subunit gene. Hence, all polypeptides of the phycobilisome (including perhaps the anchor polypeptide) may be derived from an early ancestor phycobiliprotein subunit, which itself is also related to myoglobin (Schirmer et al. (1985) J. Mol. Biol. 184, 251-277).