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Assembly of tailed bacteriophages and herpesviruses starts with formation of procapsids (virion precursors without DNA). Scaffolding proteins (SP) drive assembly by chaperoning the major capsid protein (MCP) to build an icosahedral lattice. Here we report near-atomic resolution cryo-EM structures of the bacteriophage SPP1 procapsid, the intermediate expanded procapsid with partially released SPs, and the mature capsid with DNA. In the intermediate state, SPs are bound only to MCP pentons and to adjacent subunits from hexons. SP departure results in the expanded state associated with unfolding of the MCP N-terminus and straightening of E-loops. The newly formed extensive inter-capsomere bonding appears to compensate for release of SPs that clasp MCP capsomeres together. Subsequent DNA packaging instigates bending of MCP A domain loops outwards, closing the hexons central opening and creating the capsid auxiliary protein binding interface. These findings provide a molecular basis for the sequential structural rearrangements during viral capsid maturation.

Bacillus subtilis bacteriophage SPP1 is a lytic siphovirus first described 50 years ago. Its complete DNA sequence was reported in 1997. Here we present an updated annotation of the 44,016 bp SPP1 genome and its correlation to different steps of the viral multiplication process. Five early polycistronic transcriptional units encode phage DNA replication proteins and lysis functions together with less characterized, mostly non-essential, functions. Late transcription drives synthesis of proteins necessary for SPP1 viral particles assembly and for cell lysis, together with a short set of proteins of unknown function. The extensive genetic, biochemical and structural biology studies on the molecular mechanisms of SPP1 DNA replication and phage particle assembly rendered it a model system for tailed phages research. We propose SPP1 as the reference species for a new SPP1-like viruses genus of the Siphoviridae family. [ABSTRACT FROM AUTHOR]