BACTERIOPHAGE-P2 AND BACTERIOPHAGE-P4 MORPHOGENESIS - STRUCTURE AND FUNCTION OF THE CONNECTOR

The connector, the structure located between the basteriophage capsid and tail, is interesting from several points of view. The connector is in many cases involved in the initiation of the capsid assembly proce ss, functions as a gate for DNA transport in and out of the capsid, an d is, as implied by the name, the structure connecting a tail to the c apsid. Occupying a position on a 5-fold axis in the capsid and connect ed to a coaxial 6-fold tail, it mediates a symmetry mismatch between t he two. To understand how the connector is capable of all these intera ctions its structure needs to be worked out. We have focused on the ba cteriophage P2/P4 connector, and here we report an image reconstructio n based on 2D crystalline layers of connector protein expressed from a plasmid in the absence of other phage proteins. The overall design of the connector complies well with that of other phage connectors, bein g a toroid structure having a conspicuous central channel. Our data su ggests a 12-fold symmetry, i.e., 12 protrusions emerge from the more c ompact central part of the structure. However, rotational analysis of single particles suggests that;:here are both 12- and 13-mers present in the crude sample. The connectors used in this image reconstruction work differ from connectors in virions by having retained the amino-te rminal 26 amino acids normally cleaved off during the morphogenetic pr ocess. We have used different late gene mutants to demonstrate that th is processing occurs during DNA packaging, since only mutants in gene P, coding for the large terminase subunit, accumulate uncleaved connec tor protein. The suggestion that the cleavage might be intimately invo lved in the DNA packaging process is substantiated by the fact that th e fragment cleaved off is highly basic and is homologous to known DNA binding sequences. (C) 1998 Academic Press.