The invertible segment of bacteriophage Mu DNA determines the adsorption properties of Mu particles

Abstract

INTEREST in bacteriophage Mu stems from the highly promiscuous insertion of its DNA into the genome of its host bacterium Escherichia coli (see ref. 1 for review). There are two characterstic features of Mu DNA: first, mature Mu DNA contains heterogeneous host sequences at both ends^2,3, and second, near the right end, the S end, of Mu DNA there is a 3,000 base pair sequence that can undergo inversion. The structure of Mu DNA is diagrammed in Fig. 1. The invertible sequence, called the G segment, is remarkable in that it is also found in bacteriophage P1 (ref. 4). The inversion of the G segment in Mu occurs in the prophage state and is independent of the recA function of the host⁵. According to Hsu and Davidson⁶, the inversion occurs by recombination between identical but inverted sequences of about 50 base pairs flanking the G segment. Allet and Bukhari⁷ have presented evidence that a Mu function, located within or close to the G segment, is required for the inversion reaction. When Mu particles are grown by induction of a lysogen, about half of the particles contain DNA with one orientation (referred to as the + or flip orientation) and the rest of the particles have the G segment in the reverse orientation (the − or flop orientation). However, when Mu particles are grown by infection, almost all of the particles contain DNA with the flip orientation. The predominance of the flip orientation is obtained even if the phage lysate used for infection contained equal numbers of the flip and the flop orientations. We show here that the predominance of the flip orientation after infection results from the inability of the flop containing particles to adsorb properly to the bacterial cells.