Suppression of acetate mutants in Coprinus : I. Identification of two isoaccepting tRNA suppressors of a missense mutation.

The acu-1 gene of Coprinus is the structural gene for acetyl-CoA synthetase. Cell free extracts of acu-1.4 mutants lack detectable enzyme activity. Two recessive informational suppressor genes were identified, supa4.1(+) and supa4.2(+) mapping <1.0 and 12.0 units from acu-1 respectively. Comparison of growth of suppressed mutants on selective (acetate) and non-selective (glucose) media indicated that supa4.1(+) was more efficient than supa4.2(+) but both restored an identically temperature sensitive enzyme function. Measurement of enzyme activity in cell free extracts showed that supa4.1(+) restored 29% wild type enzyme activity whereas supa4.2(+) restored 42%. Km for acetate of suppressed enzymes was indistinguishable from wild type but both had an identical but much reduced half life at 45 °C compared with wild type. These observations are consistent with supa4.1(+) and supa4.2(+) being tRNA suppressors derived by mutation in two genes encoding isoaccepting tRNAs.