G inversion in bacteriophage Mu: a novel way of gene splicing

Abstract

The G region of bacteriophage Mu is a 3,000-base pair (bp) invertible DNA segment that determines the host specificity of the phage^1,2. Only phage with the G segment in the orientation designated (+) can adsorb to Escherichia coli K-12 (refs 3–5), whereas phage with the G segment in the (−) orientation infects other hosts such as Citrobacter freundii and E. coli C (ref. 1). It has been proposed that four genes are involved in host specificity—S, U, S′ and U′—of which S and U are expressed in the orientation (+) and S′ and U′ in the inverted orientation (−) of the G segment. We have presented two alternatives for the organization of these genes (Fig. 1). In model A the four genes involved are entirely contained within the G segment, whereas in the alternative model (B) it is assumed that the proximal part of the S gene is located in the a part of the Mu genome. We report here that the S and S′ genes are partially located outside the G segment and share a common part located in α (Sc). This common S region will be joined by G inversion to one of the two variable parts (Sv or Sv′) which results in the synthesis of either tail fibre protein S or S′.