Involvement of phage Mu-1 early functions in Mu-mediated chromosomal rearrangements

Abstract

THE temperate iphage Mu-1 not only integrates at any location in the chromosome of its host Escherichia coli¹ but also provokes different types of chromosomal aberrations: insertions of extrachromosomal circular DNA^2–4, translocations, deletions and inversions of chromosomal segments (refs 5, 6, Howe and Bukhari, unpublished results and M.F., in preparation). Immunity to phage Mu prevents the induction of such events, indicating that Mu gene expression is required for these rearrangements. Furthermore, the direct participation of Mu DNA in the generation of chromosomal aberrations is demonstrated by the observations that one entire Mu genome is always adjacent to the site of the deletions, two entire Mu genomes in the same orientation flank the inserted and translocated DNA fragments, and inverted chromosomal segments are surrounded by two entire Mu genomes in opposite orientations (ref. 22 and M.F., in preparation). Dependence of such Mu-induced alteration on the A and B early phage functions was investigated. A⁻ mutants of Mu are unable to mediate all the events tested, while B⁻ mutants still promote deletion. Based on the results presented here, and on other data, we propose that the A function directly concerns the insertion of the phage and that the B gene codes for a replication function.