Hormone effects on transfer ribonucleic acid methylases in rat liver.

Abstract

five code for polar amino acids, whereas two encode a non-polar residue. The latter represents less than 10% of the mutable sites and, therefore, the majority of the amber mutants isolated should not be sensitive to amino-acid substitutions. However, out of a potential of 475 suppressed derivatives, only 240 are viable; almost half of the amino-acid substitutions are lethal. This probably reflects the complexity of the subunit interactions in the oligomeric complex and the essential role of the enzyme in cellular growth. The 240 viable species fall into nine suppression classes (V. Nene & R. E. Glass, unpublished work). Class A mutants (accept serine, glutamine, tyrosine, glutamine/tryptophan), for example. are less restrictive than Class I strains (accept only glutamine/tryptophan). Such restrictive mutants putatively indicate sites on the P-polypeptide important for function. Although the number of amber strains that accept serine outnumbers the calculated number of serine mutable sites on 8 (Table l), the majority of these amino-acid substitutions are neutral (V. Nene & R. E. Glass, unpublished work). The aromatic side group of the bulky amino acid tyrosine may explain why few sites accept this residue. In addition to the nature of the residue inserted, viability is also dependent on suppressor efficiency. Thus a poor nonsense suppressor may not be able to maintain a concentration of active RNA polymerase molecules necessary for growth. This may explain the low number of amber derivatives suppressed by SuS, since ochre suppressors function at low efficiency at amber codons (reviewed in Steege & Soll, 1979). It is tempting to suggest that survival of mutant strains with Su7 but not with Su2 is due to the insertion of tryptophan. Alternatively, this may reflect the poor suppressor efficiency on the part of Su2, as we have shown Su7 to be a consistently stronger suppressor of premature translational termination (V. Nene & R. E. Glass, unpublished work). Note that Su7 derivatives should synthesize a heterogeneous population of P-polypeptides. We have generated a family of RNA polymerase molecules by virtue of nonsense suppression of amber lesions in the P structural gene. The position of these lesions, together with the suppressor-dependent phenotype of the mutant strains, should allow determination of functional domains on the P-polypeptide.