Electiveness of photorepair, influence of dark-repair on shape of dose-response curves, and high-dose decline, in UV-induced colour mutations of Serratia

Kaplan, R.W.

Mutation Research 49(3): 357-370


ISSN/ISBN: 0027-5107
PMID: 345116
DOI: 10.1016/0027-5107(78)90106-9
Accession: 068526699

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Strain CV of S. marcescens mutates by UV with high frequency to 3 groups of mutants (w, h, s) differing in color from the red wild-type. The mutational dose-response curve has a curvature corresponding to about 3 hits. It reaches a peak and declines at high doses. Inactivation curves have a broad shoulder and mostly, but not always, a break to a lesser slope at UV doses near the peak of mutations. Photoreactivation (PR) gives a dose reduction of about 2 for both inactivation and mutation including the break and peak. The dose curve with PR for w-mutations shows 1 hit; the other types 2 hit curvature leading to a change of mutation spectrum with dose due to PR. The UV-sensitive mutant uvs21 of CV has a survival curve with a small shoulder and a long upward concavity without a break, and the mutation curve is of the 1 hit type without a peak and decline. PR gives a dose reduction of 12 for inactivation and of 7.5 for mutation. The 3 hit mutation curve of CV is interpreted by assuming that 2 further hits are required to protect the 1 hit pre-mutations from being abolished by the repair lacking in uvs21. UV induction of SOS repair cannot be responsible for the 3 hit curvature because UVR of phages and induction of prophage are already saturated at rather low doses. High-dose decline (HDD) of mutations in CV is probably caused neither by a fraction of UV-resistant cells in the population nor by post-mutational selective inhibition of growth of mutants by UV-inactivated neighbour cells. As HDD is not observed in uvs21, possibly the non-mutagenic repair lacking from uvs21 interferes with the mutation finishing processes at high doses in the reapir-proficient strain CV. UV induction of this interference cannot be a 1 hit process but requires a very large number of hits.