THE SPP1 CONNECTION

The connector of the virulent Bacillus subtilis bacteriophage SPP1 (St yloviridae) is a structure localized at the phage head vertex which at taches the tail. It is formed by oligomerization of SPP1 gene product 6 (gp6; portal protein). The purified protein is found in solution ess entially as a homo-tredecamer. Its assembly pattern resembles the turb ine-like organization found for other portal proteins and has a define d handedness (Dube et al. (1993) EMBO J. 12, 1303-1309). A preliminary reconstruction of the structure shows that gp6 is composed of a lower ring connected by a narrow region to the upper area consisting of 13 lobes radiating from an inner ring. The assembly is organized around a central channel which spans its full height. A functional characteriz ation of gp6 mutants showed that substitutions of defined amino acids by more basic residues lead to packaging of reduced amounts of DNA int o the phage head (Tavares et al. (1992) J. Mel. Biol. 225, 81-92). Sin ce SPP1 encapsidates its DNA by a headful mechanism, these mutations ( sit) affect most probably a function on the headful sensor-signal tran sduction-headful cut system. Combination of sit alleles has severe eff ects in packaging. The resulting gp6 versions lead to the encapsidatio n of shorter DNA molecules at a lower efficiency than single sit mutan ts. Gene 6 is expressed late during SPP1 infection. Interestingly, the mass of portal protein inside the cell then increases continuously un til lysis, reaching a level several fold higher than the amount requir ed to accomplish its role as a structural component of the virion.