Molecular reorganization in the hexagon to star transition of the baseplate of bacteriophage T4

The baseplate of bacteriophage T4 is a complex structure containing at least 14 different structural proteins. It undergoes a transition from a hexagonal to a star-shaped configuration during infection of the host bacterial cell. We have used a combination of genetics and image processing of electron micrographs to analyse both the wild-type structure and a series of mutant structures lacking specific gene products. Besides describing the basic anatomy of the hexagon and star configurations, we have been able to locate the products of genes 9, 11 and 12.Gene 9 product occupies a peripheral position in hexagons and stars consistent with its providing a binding site for the long tail fibres. Gene 11 product in the hexagon forms the distal part of the tail pin, which folds out to form the point of the hexagram in the star configuration. Gene 12 product is visualized as an extended 350 Å fibre in stars and broken baseplates but appears to have a more compact configuration in hexagons and intact phage.We demonstrate the structural relationship between the hexagonal and starshaped configurations and show how the positions of the specific gene products alter as a result of the structural transition. We suggest a speculative model for the role of gene 9 and gene 12 products in triggering the rearrangement of the baseplate and tail contraction.     

Plasticity of the gene functions for DNA replication in the T4-like phages

We have completely sequenced and annotated the genomes of several relatives of the bacteriophage T4, including three coliphages (RB43, RB49 and RB69), three Aeromonas salmonicida phages (44RR2.8t, 25 and 31) and one Aeromonas hydrophila phage (Aeh1). In addition, we have partially sequenced and annotated the T4-like genomes of coliphage RB16 (a close relative of RB43), A. salmonicida phage 65, Acinetobacter johnsonii phage 133 and Vibrio natriegens phage nt-1. Each of these phage genomes exhibited a unique sequence that distinguished it from its relatives, although there were examples of genomes that are very similar to each other. As a group the phages compared here diverge from one another by several criteria, including (a) host range, (b) genome size in the range between approximately 160 kb and approximately 250 kb, (c) content and genetic organization of their T4-like genes for DNA metabolism, (d) mutational drift of the predicted T4-like gene products and their regulatory sites and (e) content of open-reading frames that have no counterparts in T4 or other known organisms (novel ORFs). We have observed a number of DNA rearrangements of the T4 genome type, some exhibiting proximity to putative homing endonuclease genes. Also, we cite and discuss examples of sequence divergence in the predicted sites for protein-protein and protein-nucleic acid interactions of homologues of the T4 DNA replication proteins, with emphasis on the diversity in sequence, molecular form and regulation of the phage-encoded DNA polymerase, gp43. Five of the sequenced phage genomes are predicted to encode split forms of this polymerase. Our studies suggest that the modular construction and plasticity of the T4 genome type and several of its replication proteins may offer resilience to mutation, including DNA rearrangements, and facilitate the adaptation of T4-like phages to different bacterial hosts in nature.