Abstract— The influence of amino acid prestarvation on both the resistance to u.v. light and excision of thymine dimers of bacterial strains E. coli B/r hcr+ thy‐ trp‐, E. coli B/r hcr‐thy‐trp‐, and E, coli 15 T‐ 555–7 thy‐meth‐trp‐arg‐ has been studied. The prestarvation increased the resistance of all the strains but reasonably inhibited excision of thymine dimers. Thus the enhancement of u.v. resistance after amino acid prestarvation was not due to more complete excision of thymine dimers.

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Involvement of UV-inducible protein(s) in repair of various E. coli K-12 cell strains has been investigated using a procedure of double UV irradiation and postincubation with chloramphenicol. From the course of dose survival curves the following conclusions concerning significance of a UV-inducible protein have been drawn: 1. It is a very important for wild type cells; in these cells its early occurrence is necessary to prevent killing. 2. It is involved in repair of excision-deficient cells; however, its action early after UV is less urgent. 3. It is not involved at all in repair of lex mutant cells; 4. It exhibits some effect on survival of recA as well as recB mutant cells. We conclude that the protein is involved in excision repair as well as in resumption of DNA replication.

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Abstract— The influence of amino acid prestarvation on both the resistance to u.v. and the postirradiation repair synthesis of E. coli 15 T‐ 555‐7 thy meth arg trp and E. coli B/r (HCR+) was followed. Prestarvation increased the number of survivors about 30–100 fold in both strains at doses 600‐1200ergs/mm2. In contrast to survival no increase in repair synthesis was observed. Thus, the increase in survival has to be brought about by a mechanism which seems to be independent of additional repair synthesis.