Cloning and sequencing of the genes encoding the α and β subunits of C-phycocyanin from the cyanobacterium Agmenellum quadruplicatum

Abstract

Synthetic oligonucleotide probes were used to identify a cloned DNA fragment from the cyanobacterium Agmenellum quadruplicatum that contains the genes for the α and β subunits of C-phycocyanin. The coding region for the α-subunit gene begins 108 base pairs downstream from the 3' end of the β-subunit structural gene. The sequences of the coding regions for both genes have been determined as well as 379 base pairs of 5' flanking region, 204 base pairs of 3' flanking region, and the 108 base pairs between the two genes. The site of transcriptional initiation is located ≃325 base pairs upstream from the β-subunit gene, and an open reading frame 114 base pairs long is found within this region. The significance of this additional open reading frame is not yet known. The derived amino acid sequences for both C-phycocyanin subunits were compared with other known C-phycocyanin sequences for homology. Homologies between the A. quadruplicatum α subunit and α subunits from other species were ≃70%, as were homologies between the A. quadruplicatum β subunit and other β subunits. Homologies between the various α and β subunits were 21%-27%. Codon usage for both the C-phycocyanin α- and β-subunit genes shows asymmetries for many amino acids that correspond closely to those seen in highly expressed Escherichia coli genes.