Differential induction and repair of ultraviolet damage leading to true revesions and external suppressor mutations of an ochre codon in Escherichia coli B-r WP2.

UCH of the published work on the mechanisms by which radiation-induced lesions result in genetic mutations in bacteria has involved the reversion to prototrophy of amino-acid requiring strains, in particular the tryptophan auxotroph Escherichia coli B/r WP2 isolated by WITKIN. It has been realised for some time that mutant prototrophs are not all true reversions to ‘wild type. HILL (1963), for example, foiund that some spontaneous and ultraviolet-induced prototrophs of B/r WP2 grew more slowly and were unstable. She suggested that they might be suppressor mutants. WITKIN (1963) also gave evidence that suppressor mutations, capable of cvvercoming at a single step auxotrophies at several loci, were induced by ultraviolet light (UV) under certain conditions. Unfortunately the difficulty in performing genetic analysis in the B family of E. coli has hitherto precluded a study of the involvement of true reversion and suppressor mutation in the several phenomena associated with radiation mutagenesis. Recently BRIDGES, DENNIS and PVIuNsoN (1967) showed that many prototrophs derived from B/r WP2 contain’ed suppressors capable of suppressing chain terminating codons in T4 phage, and a simple way of distinguishing suppressor mutants from true revertants was described. (Chain terminating codons UGA, UAG (“amber”) and UAA (“ochre”) appear to code for no amino acid and thus prevent the assembly of the polypeptide chain beyond the point at which they are situated in the messenger RNA.) In this paper we show that the suppressors in B/r WP2 are all ochre suppressors from which we infer thLat the parent B/r WP2 has an ochre (UAA) codon at a tryptophan locus. A sindar conclusion has been drawn by OSBORN and PERSON (1967). We also investigate the induction of true and suppressor mutations by UV under different conditions, and compare their relative susceptibility to various processes which have been described for the removal (or bypassing) of genetic damage.