Suppressor and novel mutants of bacteriophage T4 tRNA^Gly

We have isolated a weak UGA suppressor of phage T4 tRNA^Gly in which the anticodon is changed from UCC to UCA. Two secondary mutants lacking suppressor activity are atypical in accumulating tRNA^Gly. Both mutations change the T stem of the cloverleaf model. One involves a G to A change at the 5′ base position of the middle base-pair; the second involves a C to U change at a constant base position next to the T loop. The precursor RNAs of the mutants were cleaved in vitro with the catalytic RNA subunit of RNase P. Relative to normal precursor RNA, the precursor mutated at the middle base-pair position of the T stem was cleaved more rapidly, whereas the precursor mutated at the base-pair position next to the T loop was cleaved more slowly.