In most eukaryotes the subunit 2 of cytochrome c oxidase (COX2) is encoded in intact mitochondrial genes. Some green algae, however, exhibit split cox2 genes (cox2a and cox2b) encoding two polypeptides (COX2A and COX2B) that form a heterodimeric COX2 subunit. Here, we analyzed the distribution of intact and split cox2 gene sequences in 39 phylogenetically diverse green algae in phylum Chlorophyta obtained from databases (28 sequences from 22 taxa) and from new cox2 data generated in this work (23 sequences from 18 taxa). Our results support previous observations based on a smaller number of taxa, indicating that algae in classes Prasinophyceae, Ulvophyceae, and Trebouxiophyceae contain orthodox, intact mitochondrial cox2 genes. In contrast, all of the algae in Chlorophyceae that we examined exhibited split cox2 genes, and could be separated into two groups: one that has a mitochondrion-localized cox2a gene and a nucleus-localized cox2b gene ("Scenedesmus-like"), and another that has both cox2a and cox2b genes in the nucleus ("Chlamydomonas-like"). The location of the split cox2a and cox2b genes was inferred using five different criteria: differences in amino acid sequences, codon usage (mitochondrial vs. nuclear), codon preference (third position frequencies), presence of nucleotide sequences encoding mitochondrial targeting sequences and presence of spliceosomal introns. Distinct green algae could be grouped according to the form of cox2 gene they contain: intact or fragmented, mitochondrion- or nucleus-localized, and intron-containing or intron-less. We present a model describing the events that led to mitochondrial cox2 gene fragmentation and the independent and sequential migration of cox2a and cox2b genes to the nucleus in chlorophycean green algae. We also suggest that the distribution of the different forms of the cox2 gene provides important insights into the phylogenetic relationships among major groups of Chlorophyceae.

中文翻译：

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叶绿体绿藻（Chlorophyta）中线粒体cox2基因的谱系特异性片段化和核重定位。

在大多数真核生物中，细胞色素C氧化酶（COX2）的亚基2在完整的线粒体基因中编码。但是，某些绿藻表现出分裂的cox2基因（cox2a和cox2b），它们编码形成异源二聚体COX2亚基的两个多肽（COX2A和COX2B）。在这里，我们分析了从数据库（22个分类单元中的28个序列）和这项工作中产生的新cox2数据（18个分类单元中的23个序列）获得的39个系统进化绿藻中完整和分裂的cox2基因序列的分布。我们的结果支持以前基于较少数量的分类单元的观察结果，表明类藻类，藻类和藻类中的藻类含有正统的完整线粒体cox2基因。相比之下，我们检查的藻类中的所有藻类均显示了cox2基因分裂，并且可以分为两组：一组具有线粒体定位的cox2a基因和一个核定位的cox2b基因（“ Scenedesmus-like”），另一组在核中同时具有cox2a和cox2b基因（“ Chlamydomonas-like”） ）。使用五种不同的标准推断cox2a和cox2b基因分裂的位置：氨基酸序列差异，密码子使用（线粒体相对于核），密码子偏好（第三位置频率），编码线粒体靶向序列的核苷酸序列的存在和剪接内含子。可以根据它们所包含的cox2基因的形式对不同的绿藻进行分组：完整或片段化，线粒体或细胞核定位，含内含子或无内含子。我们提出了一个模型，描述了导致线粒体cox2基因片段化以及cox2a和cox2b基因向叶绿藻绿藻中的核独立而连续迁移的事件。我们还建议，cox2基因不同形式的分布为主要的绿藻科之间的系统发育关系提供了重要的见识。