Summary: mutants 1-1 and 3 differ from the wild-type and from other UV-sensitive mutants in their behaviour after transfer from medium containing 1,2:3,4-diepoxybutane (DEB) to DEB-free medium. In both mutants several post-treatment cell cycles proceed in the absence of cell wall separation, resulting in the formation of multicellular chains or aggregates. In this study, electron and light microscopy revealed that at least one post-treatment budding cycle is accompanied by nuclear division while subsequent cell cycles can proceed in the absence of regular nuclear cycles. At low percentage survival levels, the first post-treatment budding cycle was not delayed and was accompanied by significant incorporation of radioactivity into DNA and protein. In contrast, subsequent cell cycles were found to be accompanied by only protein synthesis and not DNA synthesis. The wild-type strain, unlike the mutants, responded to DEB treatment by a dose-dependent lag in the onset of macromolecular synthesis and cell proliferation, and after prolonged incubation in mutagen-free medium the culture consisted of single budded and unbudded cells.


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