Suppressible four-base glycine and proline codons in yeast.

Five ICR-170--induced mutations at the His4 locus in yeast are +1 G.C (G, guanine; C, cytosine) additions in DNA regions that contain multiple G.C base pairs. These mutations represents both nonsuppressible and suppressible alleles. All externally, suppressible frameshift mutations occur in glycine and proline codons to produce the four-base codons GGGU (U, uracil), GGGG, and CCCU. This implies that suppression of these four-base codons in yeast, as in bacteria, involves a four-base anticodon or its functional equivalent. Two identical four-base codons (CCCU) at widely separate regions with His4 are not suppressed equally.