Disease relevance of 21S rRNA

• We propose that Mrm2p belongs to a new class of three eukaryotic RNA-modifying enzymes and is the orthologue of FtsJ/RrmJ, which methylates a nucleotide of the peptidyl transferase centre of Escherichia coli 23S rRNA that is homologous to U(2791) of 21S rRNA [1].

High impact information on 21S rRNA

• Surprisingly, little mature 21S rRNA accumulates in those cells; instead, unligated 21S rRNA exons appear [2].
• The optional 1143 bp intron in the yeast mitochondrial 21S rRNA gene (omega +) is nearly quantitatively inserted in genetic crosses into 21S rRNA alleles that lack it (omega -) [3].
• A 1.1 kb intron containing an open reading frame (ORF) in one allele (omega+) of the yeast mitochondrial 21S rRNA gene is nearly quantitatively inserted in crosses into a 21S rRNA allele lacking that intron (omega-) [4].
• The intron of the mitochondrial 21S rRNA gene of Saccharomyces cerevisiae possesses a long internal reading frame (ORF) that is conserved in various yeast species [5].
• By comparing the DNA sequence of the region of the intervening sequence in an omega+ strain with the corresponding sequence in S. carlsbergensis, we have determined the splice points of the 21S rRNA gene [6].

Biological context of 21S rRNA

• Sequence of the intron and flanking exons of the mitochondrial 21S rRNA gene of yeast strains having different alleles at the omega and rib-1 loci [7].
• This transcript contains sequences homologous to those of the mature 21S rRNA, to the intervening sequence present in the gene, and to additional sequences at the 3' end of the molecule [8].
• In vegetative diploids, chloroplast and mitochondrial genomes are transmitted biparentally, but a 1-kilobase insert present in the C. smithii mitochondrial genome spreads unidirectionally to all C. reinhardtii genomes in a manner reminiscent of the intron found in the mitochondrial 21S rRNA gene of omega + strains of yeast [9].
• Unlike a similar gene conversion event involving the insertion of an optional 1143-base-pair intron, the mitochondrial 21S rRNA gene, which requires the action of a protein encoded by a gene within that intron, conversion of the var1 GC cluster does not require any protein product of the mitochondrial genome [10].
• In omega- strains (not containing this intervening sequence) also a longer form of the 21S rRNA could be observed [11].

Anatomical context of 21S rRNA

• Thus, the maturation of 21S rRNA, a process which includes trimming at the 3' end and, in omega+ strains, the excision of a 1.1 kb intervening sequence, can occur in isolated mitochondria and appears to be dependent on ATP [12].
• We have studied a mutation (cs23) in the mitochondrial gene for 21S rRNA that affects the peptidyl transferase center of the ribosome and conditionally blocks the assembly of the 54S ribosomal subunit [13].

Associations of 21S rRNA with chemical compounds

• The complete nucleotide sequence has been determined for the intron, its junctions and the flanking exon regions of the 21S rRNA gene in three genetically characterized strains differing by their omega alleles (omega+, omega- and omega n) and by their chloramphenicol-resistant mutations at the rib-1 locus [7].
• Unlike cytoplasmic rRNA, which is highly modified, mitochondrial rRNA contains only three modified nucleotides: a pseudouridine (Psi(2918)) and two 2'-O-methylated riboses (Gm(2270) and Um(2791)) located at the peptidyl transferase centre of 21S rRNA [1].
• Two independent erythromycin resistance mutations, ER514 and ER221, have been identified in the mitochondrial gene coding for the 21S ribosomal RNA [14].

Other interactions of 21S rRNA

• The transcripts of the genes for tRNAThrACN and tRNACys (the tRNA genes immediately downstream from the 21S rRNA gene) have been analyzed; the possibility that these species represent primary transcripts is considered, and potential sites for initiation of transcription of the clustered tRNA genes are discussed [15].
• This enabled us to identify where in the determined DNA sequence the 21S ribosomal RNA and the precursor for 15S ribosomal RNA (15.5S rRNA) start, since both transcripts are initiated de novo (Levens et al. (1981) J.Biol.Chem., 256, 5226-5232) [16].

References