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MBE Advance Access published online on August 29, 2003
Molecular Biology and Evolution, doi:10.1093/molbev/msg233
Molecular Biology and Evolution © Society for Molecular Biology and Evolution 2003; all rights reserved
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Accepted July 30, 2003
© 2003 Society for Molecular Biology and Evolution

Original Articles

PCP Gene Family in Symbiodinium from Hippopus hippopus: Low Levels of Concerted Evolution, Isoform Diversity and Spectral Tuning of Chromophores

Jay R. Reichman 1*, Thomas P. Wilcox 1, and Peter D. Vize 2

1 School of Biological Sciences, University of Texas at Austin, Austin, Texas 78712, USA
2 Department of Biological Science, University of Calgary, Calgary, AB T2N 1N4, Canada

* To whom correspondence should be addressed. E-mail: reichman.jay{at}epamail.epa.gov.


  Abstract

Photosynthetic dinoflagellates have evolved unique water-soluble light harvesting complexes known as peridinin-chlorophyll a-binding proteins (PCPs). Most species of dinoflagellates express either 14-17 & 32-35 kDa mature PCP apoproteins and do so in stable combinations of isoforms that differ in isoelectric point (pI). The source (post-translational modification, protein degradation or genetic) and functional significance of PCP isoform variation have remained unclear. PCPs are encoded by multi-gene families. However, previous reports conflict over the diversity of PCP genes within gene arrays. We present the first genomic characterization of the PCP gene family from a symbiotic dinoflagellate. Symbiodinium from the Pacific bivalve Hippopus hippopus (203) contains genes for 33 kDa PCP apoproteins that are organized in tandem arrays like those of free-living dinoflagellates Amphidinium carterae, Lingulodinium (Gonyaulax) polyedra and Heterocapsa pygmaea. The Symbiodinium 203 PCP cassette consists of 1098 bp coding regions separated by approximately 900 bp spacers. The spacers contain a conserved upstream sequence similar to the promoter in L. polyedra. Surprisingly, sequences of cloned coding regions are not identical, and can differ at up to 2.2% of the nucleotide sites. Sequence variation is found at both silent and non-silent sites, and analysis of cDNA clones indicate that the variation is present in the mRNA pool. We propose that this variation represents nucleotide diversity among PCP gene copies that are evolving under low level concerted evolution. Interestingly, the predicted proteins have pIs that are within the range of those published for other species of Symbiodinium. Thus, post-translational modifications are not necessary to explain the multiple PCP isoforms. We have also identified several polymorphic sites that may influence spectral absorption tuning of chromophores.

Key Words: peridinin, PCP, gene family, dinoflagellate, Symbiodinium, concerted evolution


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