T3/T7 DNA Packaging

Abstract

During formation of a mature bacteriophage particle, a procapsid of protein packages the linear double-stranded DNA genome of the related bacteriophages, T3 and T7. Initiation of T3/T7 DNA packaging in vivo occurs near the genetic right end of a concatemer-associated genome. Initiation in vivo requires transcription near the initiation site and results in the formation of the genome’s right end by cleavage of the concatemer. The initiation is co-operative among capsids that are packaging the genomes of a single concatemer, based on observations of single in vitro DNA packaging events. An ATP fueled DNA packaging motor then drives the mature DNA genome into a capsid. Genetic analysis of a T3 DNA packaging ATPase reveals a capsid-binding site and also ATP binding sites that appear to be part of the DNA packaging motor. The packaged T7 DNA molecule is wound in concentric coils around an internal protein cylinder. The cylinder has a strong component of 8-fold rotational symmetry and is mounted on a 12-fold symmetric connector that sits on a 5-fold vertex of the procapsid. The DNA molecule enters the capsid through an axial hole in the cylinder-connector. The procapsid has recently been isolated in an altered state. In vitro T3 DNA packaging has been achieved in a purified system that has been used to define the following interaction among components: connector-large accessory protein (ATPase)-small accessory protein-DNA. Procedures are being developed for analysis of the cycle of the T3/T7 DNA packaging motor at the level of a single motor.