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.