title: Partial methylation of two adjacent adenosines in ribosomes from Euglena gracilis chloroplasts suggests evolutionary loss of an intermediate stage in the methyl-transfer reaction.
abstract: Bacterial, cytoplasmic and organellar ribosomes from a wide phylogenetic spectrum of organisms have a characteristic m6(2)Am6(2)A structure near the 3' end of the RNA of the small ribosomal subunit (SSU). We have studied one of the few exceptions to this extremely conserved post-transcriptionally modified sequence, i.e. dimethylation of only one of the two A's in chloroplasts from Euglena gracilis. It was established that only the A closest to the 5' end is dimethylated, the other one being unmodified. The methylation reaction was studied in vitro using ribosomes from a kasugamycin resistant mutant (ksgA) of Escherichia coli and purified methyl-transferase. Using limited amounts of the methyl donor S-adenosylmethionine (SAM) a partial level of methylation (50% of control) was attained. It is shown that in this case the 3' proximal A is dimethylated while the other is not. This suggests that dimethylation takes place in two successive stages. Apparently in E. gracilis chloroplasts the first stage of methylation does not occur.
mesh_terms: D000225:Adenine / Q000031:analogs & derivatives / Q000032:analysis; D000241:Adenosine / Q000032:analysis*; D005075:Biological Evolution; D002736:Chloroplasts / Q000378:metabolism*; D002989:Cloacin; D004926:Escherichia coli / Q000235:genetics; D005056:Euglena gracilis / Q000235:genetics*; D008745:Methylation; D008780:Methyltransferases / Q000378:metabolism; D012323:RNA Processing, Post-Transcriptional; D012335:RNA, Ribosomal / Q000235:genetics*; D012270:Ribosomes / Q000378:metabolism / Q000648:ultrastructure
chemical_list: D012335:RNA, Ribosomal; D002989:Cloacin; C001020:N(6),N(6)-dimethyladenine; D008780:Methyltransferases; D000225:Adenine; D000241:Adenosine