Mediates ssDNA packaging into virion, it locates to the internal surface of the capsid, thereby displacing the internal scaffolding protein B during virion formation. Protein J binds to and is packaged with the viral ssDNA. Additionally, protein J plays a role in viral attachment efficiency to the host cell. Interacts with F protein. situated at the interface between the internal surface of the capsid and the nucleic acid. Phi KhT, a host-range mutant of phi K, can grow on E.coli C and B, besides K12, and is more thermosensitive than the parental phage phi K. Belongs to the microviridae J protein family.