Nucleotide alterations in bacteriophage T4 serine transfer RNA that affect the conversion of precursor RNA into transfer RNA

Abstract

We have determined the nucleotide sequences of the serine transfer RNAs that are specified by wild-type and psu1+ amber-suppressor strains of bacteriophage T4. The two transfer RNAs have the same sequence except for their anticodons, where NGA in the wild-type species is mutated to CUA in the psu1+ species (N is a modified residue of U). These mutations are believed to confer suppressor activity on the psu1+ serine tRNA. Three secondary mutants derived from psu1+ by loss of suppressor activity have been characterized. The mutants do not synthesize serine tRNA, although they do synthesize the large precursor RNA from which serine tRNA is normally derived. Nucleotide sequence determinations of the mutant precursor RNAs show that each one differs from the psu1+ species by a nucleotide substitution that occupies either the TΨC loop (one mutant) or the adjoining base pair of the stem supporting the TΨC loop (two mutants) in the usual cloverleaf arrangement of the serine tRNA molecule. The mutant precursor RNAs are defective in the first of three steps normally associated with the conversion of precursor RNA into serine tRNA. The mutations may directly cause this defect, or the defect may result from the lack of modified nucleotides that also characterizes the mutant precursor RNAs.