The role of dimer excision in liquid-holding recovery of UV-irradiated haploid yeast.

We have directly tested the theory that liquid-holding recovery is due to an increase in the efficiency of excision-repair during holding in non-growth conditions, by assaying the dimers present in UV-irradiated cells held in saline, in growth medium or first in saline then in growth medium. We observed no differences in the amount of excision in any conditions. By assaying the kinetics of excision and comparing that with the timing of DNA synthesis, we have tested the theory that holding in growth medium allows more repair by extending the time available for it. We found that the observations were more consistent with the onset of DNA synthesis being dependent on the amount of repair rather than the converse. We have analysed the role of repair in liquid-holding recovery in a series of split-dose experiments. As Parry and Parry found, yeast cells which have been irradiated and held in non-growth conditions were much more resistant to further UV-irradiation. The increase in resistance was proportional both to the degree of fractionation of the dose and to the size of the first dose. No effect was observed if this was below 30 J . m-2. We found that the cells were able to excise more of the dimers induced if the UV dose was fractionated. We have shown that part of this increase in efficiency of excision is due to the relief of "dimer interference". "Dimer interference" is the name given to the inhibition of excision of a dimer by the presence of a neighbouring dimer. Most of the increase in efficiency, however, was due to the induction of more efficient excision repair per se, that is the excision of a greater fraction of the dimers present than could be excised in uninduced cells. Among the incidental observations we have made which are new and likely to be of interest are (1) that stationary phase cells showed a lag in the onset of excision, but log phase cells did not; (2) that excision was nevertheless constitutive in that it occurred in the presence of concentrations of cycloheximide inhibitory to protein synthesis and (3) that caffeine affected but did not inhibit dimer excision.