The bacteriophage T4 DNA packaging apparatus targets the unexpanded prohead

During the morphogenesis of the bacteriophage T4 capsid, a conformational c hange of the major head shell protein, gene product (gp) 23, causes a 50% i ncrease in capsid volume. This expansion is required to accept the full len gth chromosome and, therefore, must precede the completion of packaging. Th e expanded shell is thinner and more stable than its precursor, and can bin d accessory proteins which further stabilize it. In phages lambda, T3, T7 a nd P22, expansion occurs during DNA packaging. However, in T4, expanded cap sids can package DNA in vitro and expansion occurs in cells infected with p ackaging-defective mutants, raising the possibility that expansion and pack aging are not coupled. Proteolytically mature gp23 (gp23*) in unexpanded pr oheads is sensitive to chymotrypsin cleavage at Phe154-Ser155, creating a 3 8 kDa peptide, while gp23* in expanded capsids is refractory to the proteas e. We used an expansion assay based on this protease sensitivity to determi ne the expansion status of capsids isolated from various packaging-defectiv e mutants with the goal of determining whether packaging and expansion are normally linked. In infections at 20 degrees C, mutants in the packaging en zymes gp16 and gp17 fail to expand. However, in gene 49(-) mutants, which i nitiate packaging but fail to complete it, expansion is complete. Thus, pac kaging drives expansion, and the unexpanded prohead is the substrate for th e packaging reaction. We also show that expansion observed in 16(-) and 17(-) infections at 37 de grees C is linked to aberrant packaging. Capsids produced at 15 minutes, wh en no packaging can be detected, never expand. However, by 35 minutes when aberrant packaging begins, so does expansion of freshly made capsids. Thus in all cases now examined, expansion is only observed in vivo when DNA pack aging is also occurring, indicating that these two processes are coupled. ( C) 1998 Academic Press.