5S rRNA modification in the hyperthermophilic archaea Sulfolobus solfataricus and Pyrodictium occultum

The 5S rRNAs from Sulfolobus solfataricus and Pyrodictium occultum were digested to nucleosides and analyzed using directly-combined HPLC/mass spectrometry. P. occultum 5S rRNA contains two modified nucleoside species, N4-acetylcytidine (ac4C) and N4-acetyl-2'-O-methylcytidine (ac4Cm). Oligonucleotides were generated from P. occultum 5S rRNA by RNase T1 hydrolysis, and their molecular weights were determined using electrospray mass spectrometry and compared with those predicted from the P. occultum 5S RNA gene sequence. Deviation in mass between expected and observed molecular weights permitted ac4Cm to be located at position 35, in the nonanucleotide CAA-CACC[ac4Cm]G, and the ac4C in one or both of two (C,U)G trinucleotides. 2'-O-Methylcytidine is unambiguously characterized in S. solfataricus 5S rRNA, confirming earlier tentative assignments at the analogous sequence position (Stahl, D.A., Luehrsen, K.R., Woese, C.R., and Pace, N.R. (1981) Nucleic Acids Res., Vol. 9, pp. 6129-6137; Dams, E., Londei, P., Cammarano, P., Vandenberghe, A., and De Wachter, R. (1983) Nucleic Acids Res. Vol. 11, pp. 4667-4676). Potential effects of the presence of ac4C and ac4Cm on thermal stabilization of 5S rRNA in thermophiles are discussed.