Prevention of Translational Frameshifting by the Modified Nucleoside 1-Methylguanosine

Abstract

The methylated nucleoside 1-methylguanosine (m 1 G) is present next to the 3′ end of the anticodon (position 37) in all transfer RNAs (tRNAs) that read codons starting with C except in those tRNAs that read CAN codons. All of the three proline tRNA species, which read CCN codons in Salmonella typhimurium , have been sequenced and shown to contain m 1 G in position 37. A mutant of S. typhimurium that lacks m 1 G in its tRNA when grown at temperatures above 37°C, has now been isolated. The mutation ( trmD3 ) responsible for this methylation deficiency is in the structural gene ( trmD ) for the tRNA(m 1 G37)methyltransferase. Therefore, the three proline tRNAs in the trmD3 mutant have an unmodified guanosine at position 37. Furthermore, the trmD3 mutation also causes at least one of the tRNA Pro species to frequently shift frame when C's are present successively in the message. Thus, m 1 G appears to prevent frameshifting. The data from eubacteria apply to both eukaryotes and archaebacteria.