POSTTRANSCRIPTIONAL MODIFICATIONS IN 16S AND 23S RIBOSOMAL-RNAS OF THE ARCHAEAL HYPERTHERMOPHILE SULFOLOBUS-SOLFATARICUS

Posttranscriptional modification is common to many types of RNA, but t he majority of information concerning structure and function of modifi cation is derived principally from tRNA, By contrast, less is known ab out modification in rRNA in spite of accumulating evidence for its dir ect participation in translation. The structural identities and approx imate molar levels of modifications have been established for 16S and 23S rRNAs of the archaeal hyperthermophile Sulfolobus solfactaricus by using combined chromatography-mass spectrometry-based methods. Modifi cation levels are exceptionally high for prokaryotic organisms, with a pproximately 38 modified sites in 165 rRNA and 50 in 23S rRNA for cell s cultured at 75 degrees C, compared with 11 and 23 sites, respectivel y, in Escherichia coli, We structurally characterized 10 different mod ified nucleosides in 16S rRNA, 64% (24 residues) of which are methylat ed at O-2' of ribose, and 8 modified species in 23S rRNA, 86% (43 resi dues) of which are ribose methylated, a form of modification shown in earlier studies to enhance stability of the polynucleotide chain. From cultures grown at progressively higher temperatures, 60, 75, and 83 d egrees C, a slight trend toward increased ribose methylation levels wa s observed, with greatest net changes over the 23 degrees C range shot M for 2'-O-methyladenosine in 16S rRNA (21% increase) and for 2'-O-met hylcytidine (24%) and 2'-O-methylguanosine (22%) in 23S rRNA. These fi ndings are discussed in terms of the potential role of modification in stabilization of rRNA in the thermal environment.