An optional group I intron between the chloroplast small subunit rRNA genes of Chlamydomonas moewusii and C. eugametos

Abstract

We report the presence of a 402 bp group I intron in the chloroplast small subunit (SSU) rRNA gene of Chlamydomonas moewusii. The intron in inserted within the highly conserved '530 loop', at a site corresponding to positions 531-532 of the E. coli 16rRNA. Residues surrounding the insertion site almost certainly play an important role in ribosomal proofreading function as they proved to be protected by tRNAs in E. coli 16S rRNA (Moazed and Noller 1986; Stern et al. 1986). The C. moewusii intron revealed a secondary structure model which differs substantially from those of the typical subgroup IA and IB introns. This model, however, shows striking similarities with the structures of the C. reinhardtii chloroplast 23S rRNA gene intron (Rochaix et al. 1985), the S. cerevisiae mitochondrial COB3 intron (Holl et al. 1985) and the three introns of phage T4 in the nrdB, td and sunY genes (Shub et al. 1988). The SSU rRNA gene intron is absent from C. eugametos, an alga that is interfertile with C. moewusii. The presence/absence of the intron account for a 390 bp restriction fragment length polymorphism between the two algal SSU rRNA genes, a polymorphic locus that is strictly co-inherited with a tightly linked streptomycin resistance mutation (sr-2) in interspecific hybrids between the two algae.

The gene coding for small ribosomal subunit RNA in the basidiomyceteUstilago maydiscontains a group I intron

“…Alignment showed the extra length to be due to a 411 nucleotide insertion between the nucleotides occupying positions 1158 and 1159 counting from the 5'-terminus of the SSU rRNA. This situation is reminiscent of those described for the plastid SSU rDNA of the green alga C. moewusii (13), and for the nuclear SSU rDNAs of the fungus P. carinii (11) and the green alga A.…”

“…In nuclear SSU rDNA, the reported occurrences are in the fungus Pneumocystis carinii (1 1) and the green alga Ankistrodesmus stipitatus (12). The third case of an SSU rDNA containing a group I intron is in the plastidial genome of the green alga Chlamydomonas moewusii (13). Each of these SSU rDNAs contains a single intron, each time in a different location.…”

Evolution of Fragmented Mitochondrial Ribosomal RNA Genes in Chlamydomonas

“…b Sizes of the RNAs (distinct from the chloroplast and cytosolic LSU rRNAs) that hybridized with probe 1 (1) and with probe 2 and/or 3 (2/3) in northern blot analysis of total cellular RNA. For each taxon, the sum of the sizes of the two RNAs detected is shown in parentheses (Bourque et al 1971); 1.7 kb, chloroplast LSU ␦ fragment (Rochaix and Darlix 1982; Lemieux et al 1989;Turmel et al 1993); 1.5 kb, chloroplast SSU rRNA (Dron et al 1982;Durocher et al 1989). in size between those of the other clade members examined. This observation could be explained by changes in the size of variable regions in this portion of the LSU rRNA.…”

Gene rearrangements in Chlamydomonas chloroplast DNAs are accounted for by inversions and by the expansion/contraction of the inverted repeat

“…The significant reduction in size of both the chloroplast and mtDNAs of C. pitschmannii relative to its C. eugametos and C. moewusii counterparts indicates that the two organelle DNAs have followed parallel evolutionary pathways with regards to the direction of insertion/deletion events. As length mutations due to differences in group I intron composition have been identified in the coding regions of both chloroplast and mitochondrial genes in C. eugametos and C. moewusii [13,15,44], such mutations are probably responsible for at least part of the size difference between the C. pitschmannii and C. eugametos/C, moewusii organelle DNAs. Supporting this notion is the observation that the chloroplast large subunit rRNA gene of C. pitschmannii is not interrupted by any group I introns, whereas those of C. eugametos and C. moewusii contains six and five group I introns, respectively [44].…”