The purpose of this work was to examine the tRNA Trp -encoding genes ( tRNA Trp ) of Saccharomyces cerevisiae to gain insight as to why tRNA Trp amber suppressors, isolated by conventional genetic techniques, have not been reported. The results herein indicate that the haploid yeast genome contains six tRNA Trp genes which map to five or six chromosomes. Not only do the six genes have identical coding sequences but their introns are also identical. Gene replacement experiments indicate that five copies of tRNA Trp are sufficient for cell viability. Thus, mutation of one tRNA Trp gene to a suppressor in vivo, lowering the functional number of tRNA Trp genes, would not be expected to be lethal.
