DECREASED RATES OF REPLICON INITIATION IN MAMMALIAN-CELLS

We have designed an assay to measure the rate of initiation of DNA syn thesis in Friend erythroleukemia cells and have shown that this parame ter is reduced by gamma-radiation and treatment with llotoxin-9-(4,6-O -ethylene-beta-D-glucopyranoside) (VP-16). It is concluded, that doubl e-strand breaks in DNA are the immediate cause for this effect. The de crease in the rate of replicon initiation is affected differently by d ifferent agents such as cis-diamminedichloroplatinum(II), cycloheximid e, staurosporine, and 3-aminobenzamide. The analysis of these results indicates that the observed partial decrease of the rate of DNA initia tion is most probably transmitted from the site of damage to the initi ation site by one or more phosphorylation/dephosphorylation steps. It does not require de novo synthesis of protein factors, but is probably dependent on poly(ADP-ribosyl)ation of chromatin at the site of DNA b reaks.