Mitochondrial genes in the colourless algaPrototheca wickerhamiiresemble plant genes in their exons but fungal genes in their introns

Abstract

The mitochondrial DNA from the colourless alga Prototheca wickerhamii contains two mosaic genes as was revealed from complete sequencing of the circular extranuclear genome. The genes for the large subunit of the ribosomal RNA (LSUrRNA) as well as for subunit I of the cytochrome oxidase (coxI) carry two and three intronic sequences respectively. On the basis of their canonical nucleotide sequences they can be classified as group I introns. Phylogenetic comparisons of the coxI protein sequences allow us to conclude that the P.wickerhamii mtDNA is much closer related to higher plant mtDNAs than to those of the chlorophyte alga C.reinhardtii. The comparison of the intron sequences revealed several unusual features: (1) The P.wickerhamii introns are structurally related to mitochondrial introns from various ascomycetous fungi. (2) Phylogenetic analyses indicate a close relationship between fungal and algal intronic sequences. (3) The P. wickerhamii introns are located at positions within the structural genes which can be considered as preferred intron insertion sites in homologous mitochondrial genes from fungi or liverwort. In all cases, the sequences adjacent to the insertion sites are very well conserved over large evolutionary distances. Our finding of highly similar introns in fungi and algae is consistent with the idea that introns have already been present in the bacterial ancestors of present day mitochondria and evolved concomitantly with the organelles.

Physical map and gene organization of the mitochondrial genome of Chondrus crispus (Rhodophyta, Gigartinales)

“…To our knowledge this is the first red algal mitochondrial genome mapped to date. Its size and configuration are similar to those of the two other mitochondrial genomes which have been partially characterized from rhodophytes, the ca 28. kb and 40 kb circular mtDNA molecules of Gracilariopsis lemaneiformis [91 and Griffithsiapacifica [19], respectively.…”

“…More recently, the circular mitochondrial genomes of Chlamydomonas moewusii and Chlamydomonas eugarnetos were mapped, indicating sizes of 22 and 24 kb respectively [7]. In chlamydomonads the mitochondrial ribosomal RNA-coding regions are split into small fragments scattered among other genes of the genome whereas the Chlorococcales Prototheca wickerhamii [28] and Scenedesmus obliquus [16] harbour intron-containing mitochondrial rRNA genes more closely related to those of higher plants. Another feature of the Chlamydomonas mitochondrial genome is the absence of 5 S rRNA, contrary to what is found in green plants and in the Chlorococcales.…”

Transcript mapping and processing of mitochondrial RNA in the chlorophyte alga Prototheca wickerhamii

“…In contrast, the linear mtDNA from Chlamydomonas reinhardtii branches differently either with animal or with fungal mt sequences, showing no reliable relation to the plant mt clade [19,31]. Data from complete sequencing of the 55 328 bp mtDNA from the colourless chlorophycean alga Prototheca wickerhamii showed that mitochondria from plants and P. wickerhamii belong to the same evolutionary lineage, sharing a quite recent common ancestor [84,85].…”

“…[77,50,65,25]). Group I introns of the cox 1 gene from P. wickerhamii show structural similarities to corresponding fungal introns [84], and have therefore been chosen for comparative in vitro self-splicing experiments. Two of the three group I introns residing in the coxl gene displayed in vitro selfsplicing activity under conditions defined for splicing of structurally related fungal introns [ 68].…”

“…In the coxl gene from P. wiekerhamii there are three group I introns, which share structural features with counterparts from fungal and liverwort mitochondria [84,57].…”