Figure 1

Initiation of DNA packaging by the closely related bacteriophages, T3 and T7. (a) Initiation is illustrated for the simplest DNA packaging. This packaging has a monomeric DNA substrate and was demonstrated for T3 and assumed for T7. Packaging of this type occurs only in vitro, as far as is known (review [31,32]). (b) Initiation is illustrated for the more complex DNA packaging that occurs in vivo for both T3 and T7 (review [31,32]). In (b), the DNA substrate is an end-to-end joined concatemeric DNA for which only one monomer is completely shown. Dashed lines in (b) indicate part of another monomer within the concatemer. The following details of the concatemer are omitted for simplicity: replication forks and interaction among different procapsids (review [32]). The various proteins and protein assemblies of the initiation complex, including the connector and DNA packaging ATPase, are identified in the rectangular box. Proteins have both descriptive names and names based on gene number [2], preceded by gp. The letter, R, indicates the right end of the mature DNA molecule; the letter, L, indicates the left end.