Reichman Jay R, Wilcox Thomas P, Vize Peter D
School of Biological Sciences, University of Texas at Austin, USA.
Mol Biol Evol. 2003 Dec;20(12):2143-54. doi: 10.1093/molbev/msg233. Epub 2003 Aug 29.
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 to 17 kDa or 32 to 35 kDa mature PCP apoproteins and do so in stable combinations of isoforms that differ in isoelectric point (pI). The source (posttranslational modification, protein degradation, or genetic) and functional significance of PCP isoform variation have remained unclear. PCPs are encoded by multigene 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 1,098-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 nonsilent 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, posttranslational 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.
光合甲藻进化出了独特的水溶性光捕获复合体，即多甲藻素 - 叶绿素a结合蛋白（PCP）。大多数甲藻物种表达14至17 kDa或32至35 kDa的成熟PCP脱辅基蛋白，并且以等电点（pI）不同的同工型稳定组合形式表达。PCP同工型变异的来源（翻译后修饰、蛋白质降解或遗传）及其功能意义仍不清楚。PCP由多基因家族编码。然而，先前的报道在基因阵列中PCP基因的多样性方面存在冲突。我们展示了来自共生甲藻的PCP基因家族的首次基因组特征。来自太平洋双壳贝类波纹唇鱼（203）的共生藻含有33 kDa PCP脱辅基蛋白的基因，这些基因像自由生活的甲藻卡特亚多甲藻、多边舌甲藻（原多甲藻）和矮小异帽藻那样串联排列。共生藻203的PCP盒由1098 bp的编码区组成，编码区之间间隔约900 bp的间隔序列。间隔序列包含一个与多边舌甲藻启动子相似的保守上游序列。令人惊讶的是，克隆的编码区序列并不相同，核苷酸位点差异可达2.2%。在沉默位点和非沉默位点均发现序列变异，对cDNA克隆的分析表明该变异存在于mRNA库中。我们认为这种变异代表了在低水平协同进化下进化的PCP基因拷贝之间的核苷酸多样性。有趣的是，预测的蛋白质的pI在已发表的其他共生藻物种的pI范围内。因此，翻译后修饰并非解释多种PCP同工型所必需的。我们还鉴定了几个可能影响发色团光谱吸收调谐的多态性位点。
