Regular spliceosomal introns are invasive in Chlamydomonas reinhardtii: 15introns in the recently relocated mitochondrial cox2 and cox3 genes

In the unicellular green alga, Chlamydomonas reinhardtii, cytochrome oxidas e subunit 2 (cox2) and 3 (cox3) genes are missing from the mitochondrial ge nome. We isolated and sequenced a BAC clone that carries the whole cox3 gen e and its corresponding cDNA. Almost the entire cox2 gene and its cDNA were also determined. Comparison of the genomic and the corresponding cDNA sequ ences revealed that the cox3 gene contains as many as nine spliceosomal int rons and that cox2 bears six introns. Putative mitochondria targeting signa ls were predicted at each N terminal of the cox genes. These spliceosomal i ntrons were typical GT-AG-type introns, which are very common not only in C hlamydomonas nuclear genes but also in diverse eukaryotic taxa. We found no particular distinguishing features in the cox introns. Comparative analysi s of these genes with the various mitochondrial genes showed that 8 of the 15 introns were interrupting the conserved mature protein coding segments, while the other 7 introns were located in the N-terminal target peptide reg ions. Phylogenetic analysis of the evolutionary position of C reinhardtii i n Chlorophyta was carried out and the existence of the cox2 and cox3 genes in the mitochondrial genome was superimposed in the tree. This analysis cle arly shows that these cox genes were relocated during the evolution of Chlo rophyceae. It is apparent that long before the estimated period of relocati on of these mitochondrial genes, the cytosol had lost the splicing ability for group II introns. Therefore, at least eight introns located in the matu re protein coding region cannot be the direct descendant of group II intron s. Here, we conclude that the presence of these introns is due to the invas ion of spliceosomal introns, which occurred during the evolution of Chlorop hyceae. This finding provides concrete evidence supporting the "intron-late " model, which rests largely on the mobility of spliceosomal introns.