Headful DNA packaging: bacteriophage SPP1 as a model system

Tailed bacteriophages and herpesviruses package DNA inside the viral capsid by a powerful molecular motor. This packaging machine is composed of the portal protein, which provides a gate for DNA entry, the large terminase subunit whose ATPase activity fuels DNA translocation, and most frequently, a small terminase subunit that recognizes the viral packaging site. Here we review the mechanisms how the virulent Bacillus subtilis phage SPP1 packages DNA into a preformed procapsid. Encapsidation of the SPP1 DNA follows a processive unidirectional headful mechanism that starts with the recognition and cleavage of a unique genomic sequence (pac) by the viral terminase. The viral genome is then translocated through the central channel of the portal protein found at a single vertex of the procapsid. Packaging is terminated by an endonucleolytic cleavage of the concatemeric DNA substrate, following by disassembly of the packaging motor and closure of the portal system by the gatekeepers preventing leakage of the viral genome. Recent advances are providing new molecular insights on the mechanisms that ensure precise coordination of these critical steps required to accomplish the packaging encapsidation cycle.