Characterization of the genes encoding phycoerythrin in the red alga Rhodella violacea: evidence for a splitting of the rpeB gene by an intron.

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Abstract
The phycobilisome of the eukaryotic unicellular red alga Rhodella violacea presents in some respects an organization that is intermediate between those of the homologous counterparts found in cyanobacteria (the putative chloroplast progenitor) and more advanced, pluricellular red algae. This suggests evolutionary relationships that we investigated at the genome level. The present work describes the sequences of two rhodophytan phycobilisome genes, rpeA and rpeB. These chloroplast genes encode the alpha and beta subunits of phycoerythrin, the major component of the light-harvesting antennae and one of the most abundant cellular proteins in these algae. The amino acid sequences deduced from both rpeA and rpeB present strong homologies with those previously reported for phycoerythrin subunits of cyanobacteria, rhodophyta, and cryptomonads. The main difference with the corresponding cyanobacterial genes was the unexpected occurrence of an intervening sequence that split rpeB into two exons. This intervening sequence presents characteristics of group II introns but lacks several structural domains. Transcriptional analyses showed that the two rpe genes are cotranscribed and that the major RNA species detected corresponds to a mature mRNA lacking the intron. As the phycobiliproteins form a group of closely related polypeptides in cyanobacteria and rhodophyta, the molecular events affecting the corresponding genes, such as the rpeB intron, may be a clue to elucidate some aspects of the molecular processes involved in the evolution of plastid genes.