Interaction of restoration processes in UV irradiated Escherichia coli cells

An attempt was performed to estimate survival and course of DNA synthesis in Escherichia coli B/r hcr+ and hcr- cells in relation to the amount of unexcised dimers. In exponential growing hcr+ cells irradiated with 30 Jm-2, dimers were almost completely excised and survival of cells was equal to about 3%. In the hcr+ cells prestarved for amino acid and thymine and irradiated by the same fluence, survival of cells was almost equal while two thirds of dimers remained unexcised and could be detected in the hybrid DNA consisting of parental and daughter chains. In exponentially growing hcr- cells irradiated with 20 Jm-2, the same amount of dimers was produced which remained unexcised in the prestarved hcr+ cells. However, their survival was equal to about 0.02%. Despite the great differences in dimer contents, about one third of DNA was replicated after UV in both exponentially growing and prestarved hcr+ cells producing well defined HL-hybrid peak, and the newly synthesized DNA was normal-sized. In hcr- cells which contained approximately the same amount of dimers as in hcr+ prestarved cells, the amount of replicated DNA was too low to form a detectable density labelled hybrid peak, and the newly synthesized DNA was in short pieces. Thus, when hcr+ and hcr- cells contain the same number of residual dimers, they have different levels of tolerance to these dimers. (author)