Molecular characterization and evolution of sequences encoding light-harvesting components in the chromatically adapting cyanobacterium Fremyella diplosiphon.

Abstract

The major light-harvesting complex in eukaryotic red algae and prokaryotic cyanobacteria is the phycobilisome, a water-soluble complex located on the outer surface of the photosynthetic membranes and composed of both pigmented phycobiliproteins (85%) and non-pigmented linker (15%) polypeptides. The phycobiliproteins are encoded by a gene family and exhibit varying degrees of sequence homology (25 to 55%). Some cyanobacteria can maximize the absorption of prevalent wavelengths of light by adjusting the phycobiliprotein composition of the phycobilisome, a process called complementary chromatic adaptation. In the chromatically adapting species Fremyella displosiphon, there are at least two sets of phycocyanin genes; one is transcribed as two red light-induced transcripts and the other is encoded on a single transcript present in both red and green light. We have determined the complete nucleotide sequences of both sets of phycocyanin subunit genes and their associated 5' and 3' regulatory regions. Based on S1 nuclease protection experiments, the transcripts (1600 and 3800 bases) encoding the inducible phycocyanin subunits have the same 5' end, and possible mechanisms for their synthesis are presented. The 5' end of the 1500-base transcript encoding the constitutive phycocyanin subunits was determined and revealed an Escherichia coli-like "-10" and "-35" region, and sequences near the transcription initiation site homologous to the analogous region of the phycocyanin gene set of Anabaena sp. 7120. Determination of the 3' ends of the transcripts encoding both F. diplosiphon phycocyanin gene sets revealed regions of potential secondary structure that may be important for transcription termination and/or transcript stability. In addition, the sequence of an open reading frame (encoding a 30 kDa polypeptide), located 3' to the constitutive phycocyanin gene set in F. diplosiphon and highly conserved in at least three cyanobacterial species, is presented. The same high degree of sequence homology between the two F. diplosiphon PC alpha and PC beta sequences (85 and 77%, respectively) was found at both the nucleotide and amino acid levels, and similar results were obtained for interspecies comparisons. Implications of these homologies with regard to the evolution of phycobiliprotein subunits are discussed.