Role of recombinational repair in sensitivity to an antitumour agent that inhibits bacteriophage T4 type II DNA topoisomerase

Summary The bacteriophage T4‐encoded type II DNA topoisomerase is the major target for the antitumour agent m‐AMSA (4‐(9‐acridinylamino)methanesulphon‐m‐anisidide) in phage‐infected bacterial cells. Inhibition of the purified enzyme by m‐AMSA results in formation of a cleavage complex that contains the enzyme covalently attached to DNA on both sides of a double‐strand break. In this article, we provide evidence that this cleavage complex is responsible for inhibition of phage growth and that recombinational repair can reduce sensitivity to the antitumour agent, presumably by eliminating the complex (or some derivative thereof). First, topoisomerase‐deficient mutants were shown to be resistant to m‐AMSA, indicating that m‐AMSA inhibits growth by inducing the cleavage complex rather than by inhibiting enzyme activity. Second, mutations in several phage genes that encode recombination proteins (uvsX, uvsY, 46 and 59) increased the sensitivity of phage T4 to m‐AMSA, strongly suggesting that recombination participates in the repair of topoisomerase‐mediated damage. Third, m‐AMSA stimulated recombination in phage‐infected bacterial cells, as would be expected from the recombinational repair of DNA damage. Finally, m‐AMSA induced the production of cleavage complexes involving the T4 topoisomerase within phage‐infected cells.