English: Schematic of the bacteriophage MS2 lifecycle. Virions initially bind to the sides of the bacterial pilus via the MP. MPs are an essential single-copy structural component of RNA phages. By a mechanism that remains largely obscure, the RNA–MP complex but not the remaining capsid shell enters the cell. MP also binds to its own PSs located toward either end of the MS2 genome. Recent asymmetric reconstruction of the MS2–pilus complex suggests that MP replaces a CP dimer of the C/C conformer in an otherwise entirely icosahedral protein shell, from which position it is ideally placed to contact the cellular receptor and escort the RNA into the cytoplasm. Note that the presence of the asymmetric MP component could not be detected in averaged X-ray and EM density maps. Once internalized, the MP is cleaved into two separate fragments by protease, allowing translation and replication to start. Temporal control of phage gene expression then regulates the production of progeny genomes and structural proteins that assemble prior to the action of the phage lysis protein.