DNA repair after ultraviolet irradiation in synchronous plasmodia of Physarum polycephalum

Abstract

Plasmodia of Physarum polycephalum were irradiated with 14 500 ergs/mm 2 of ultraviolet irradiation during early G2 period and the subsequent repair synthesis of DNA was studied. It was found that the G2 period, which is normally 5 h in length, is prolonged to 18 h by such a dose of irradiation and that repair synthesis occurs almost entirely during the last 9 h of this period. Following the extended G2 period, the nuclei undergo mitosis in a normal manner and during the DNA synthesis period which follows mitosis all the nuclear DNA is replicated semiconservatively as shown by density label centrifugation studies. During the extended G2 period, normal replication of the nucleolar satellite DNA and trace amounts of nuclear DNA occurs and the total amount synthesized is equal to what would have been synthesized in a G2 period of normal length. Caffeine shows a strong inhibitory effect on DNA repair synthesis without interfering with DNA replication or the length of the intermitotic period.