Influence of thymine starvation on UV mutability of Escherichia coli B/r Hcr+ thy- trp-

Escherichia coli B/r Hcr+ thy- trp- cells were cultivated in a synthetic glucose medium supplemented with 2 μg/ml thymine and 14 μg/ml tryptophan until the beginning of the exponential growth phase. After filtration and washing the cells were thymine starved for different periods of time. During short-term starvation (about 40 minutes) the cells did not die and the frequency of Trp+ revertants as determined on the synthetic glucose medium supplemented with 2 μg/ml thymine and 0.75 μg/ml tryptophan solidified with agar did not increase. From the 45th min of starvation cells died exponentially and at the same time the fraction of Trp+ revertants in the population increased. During short-term starvation the sensitivity of cells to ultraviolet radiation become enhanced, at the same time one could see an increase of frequency of ''mutation-frequency-decline''-stable ultraviolet induced Trp+ revertants. Is is supposed that short-term thymine starvation affects the coordination of the rec+ and polAl+ systems participating in the uvr+ dependent DNA repair synthesis in favour of the rec+ system, incidentally starvation may affect the error-free postreplication repair in which the products of the uvr+ and rec+ genes participate. (author)