Nontriplet tRNA–mRNA Interactions

Abstract

Genetic decoding proceeds overwhelmingly in a triplet manner. However, studying the exceptions to this rule may help in the elucidation of the rules governing mRNA–tRNA interaction of the ribosome, the function of some modified bases in tRNA, and whether any proteins derived from nontriplet reading are utilized by cells or viruses. In addition, since it is possible that the primitive genetic code used predominately nontriplet codon–anticodon interactions (Woese 1972; Crick et al. 1976), studies on the extent of the alterations required to allow the present-day code to be read in a nontriplet manner at certain codons may provide some insight into the evolution of the code. This chapter summarizes the current status of the investigation on these topics. The first part considers mutationally produced nontriplet reading, and the second part considers the low-level nontriplet reading normally encountered at certain codons.

EXTRAGENIC SUPPRESSORS FOR FRAMESHIFT MUTANTS
Mutationally produced nontriplet reading has been sought by selecting for extragenic suppressors of frameshift mutations. The pioneering studies of Crick et al. (1961) showed that insertion or deletion of one or two bases in a gene causes the ribosomes to shift reading frame; they termed these alterations frameshift mutations. These studies revealed the general nature of the code and were confirmed by the amino acid sequencing studies of altered proteins by Streisinger et al. (1967) and also by a variety of other approaches. However, these early studies on frameshift mutations used mutant phages; it was not until Ames and Whitfield (1967) described a...