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6 F. Eckardt and R.H. Haynes, Depar tment of Biology, York University, Toronto (Canada)

Linearity of induced mutagenesis at low UV doses in yeast

We have measured the frequency (mutants per survivor) of UV-induced re- version to pro to t rophy (Ade * and Lys*) in both excision deficient and profi- cient haploid strains of yeast (Saccharomyces cerevisiae) over a relatively wide range of doses (1 to 1500 ergs/mm 2). Double log plots of these data (corrected for spontaneous revertants) show that mutat ion induction is linear at low doses. A transition to higher order kinetics occurs at doses near 15 ergs/mm 2 in ex- cisionless strains and near 70 ergs/mm 2 in repair-proficient strains. In the highest observed dose ranges the slopes of the induction curves attain values as high as 3. These results suggest that, at sufficiently low doses, UV mutagenesis in yeast is a single-event Poisson process, statistically independent of cell killing, and that the commonly reported quadratic kinetics observed at moderate doses can be usefully regarded as a departure from what is basically a linear proces. There are three possible reasons, not mutually exclusive, for the transition to higher order kinetics: first, for presently obscure macromolecular reasons, mutat ion and killing may not remain statistically independent processes as doses are in- creased; second, the UV-sensitivity of the auxotrophs might be greater than the sensitivity of the prototrophic revertants; and third, the effective cross-section for mutagenesis might itself increase with dose. The latter effect could arise either from an increasing frequency of error-prone repair events or from the switching-on of supplementary error-prone repair pathways by the radiation it- self.

W o r k s u p p o r t e d b y the D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t a n d the N a t i o n a l R e s e a r c h C o u n c i l of C a n a d a .

7 P. Karran, A. Moscona, A. Norin, D, Scudiero and B. Strauss, University of Chicago, Illinois 60637 (U.S.A.)

Non-genetic factors affecting the quantitative repair capability of cells.

Although genetic factors ultimately determine the repair capability of cells, the maximum repair rate is also fixed by the state of the particular cell type studied. The rate at which human peripheral blood lymphocytes (HPBL) are able to excise damage is only one tenth of the rate at which HPBL cultures in- cubated 72 h with the mitogen concanavalin A remove acetoxy acetyl amino fluorene (AAAF)-induced damage and only about one thirtieth of the capacity of a lymphoblastoid line derived from a Burkitt 's lymphoma. The initial rate of repair after AAAF-induced damage is greater than that after t reatment with methyl methanesulfonate (MMS) when human lymphoma cells are tested but the initial AAAF- and MMS-induced repair rates are indistinguishable when tested with HPBL not stimulated with mitogen. An interpretation of these obser-

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rations is that agents such as AAAF require a rate-limiting excision enzyme for their repair, that this enzyme is particularly deficient in non-proliferating cells and that the enzyme is not required for the repair of MMS-induced damage.

We tested one prediction of this hypothesis using developing chick retino- blasts. As differentiation proceeds these cells lose their ability to synthesize DNA. We found that loss of DNA synthetic ability is correlated with a loss of repair capacity although the kinetics of the decline differ for DNA synthesis and repair. The decline in repair cannot be accounted for by loss of thymidine kinase or DNA polymerase.

The results imply that quantitative estimates of DNA repair require specifi- cation of the physiological and developmental state of the cells used in addition to standardization of their genetic type. Insofar as repair and replication may be uncoupled, they also suggest the existence of physiological states in which cells may be particularly susceptible to mutagenesis due to a deficiency in re- pair.

This w o r k was s u p p o r t e d b y g r a n t s f r o m E R D A E ( 1 1 - 1 ) 2 0 4 0 and the Nat ional Inst i tutes o f Hea l t h , GM 0 7 8 1 6 , CA 1 4 5 9 9 - 0 3 .

8 P. Clifford, University of California, Berkely, California, 94720 (U.S.A.)

Interspecific cytogenetics: the quantitative effect of interphase chromosome disposition

A number of recent and proposed studies are concerned with interspecific comparisons of mammalian radiosensitivity using the irradiated lymphocyte system. Taking as the endpoint the total centric yield at the first mitosis fol- lowing irradiation Brewen et al. have proposed a linear interpolatory formula for the relationship between the centric yield and the number of arms in the specific karyotype. The model of Savage and Papworth provides a structural basis for a relationship which although nonlinear is in broad agreement with ob- servation.

A temptat ion exists to attr ibute deviations from the theoretical predictions to properties of the karyotype which have been ignored -- specifically the rela- tive arm lengths. The purpose of this paper is to show that under a variety of hypotheses about the disposition of the interphase chromosomes the effect of differences in relative arm lengths is minor and thus cannot be relied upon to explain exceptions to Brewen's rule.

This inves t igat ion was supp or te d in part by the auspices o f the U.S. Energy Research and D e v e l o p m e n t A g e n c y and in part by the Gese l l schaf t f u r S t r ah l en - und U m w e l t s f o r s c h u n g , N e u h e r b e r g bei Munchen .

9 K.H. Thompson and R.C. Sparrow, Biology Department, Brookhaven National Laboratory, Upton, New York 11973 (U.S.A.)