T4 genes in the marine ecosystem: studies of the T4-like cyanophages and their role in marine ecology

Abstract

From genomic sequencing it has become apparent that the marine cyanomyoviruses capable of infecting strains of unicellular cyanobacteria assigned to the genera Synechococcus and Prochlorococcus are not only morphologically similar to T4, but are also genetically related, typically sharing some 40-48 genes. The large majority of these common genes are the same in all marine cyanomyoviruses so far characterized. Given the fundamental physiological differences between marine unicellular cyanobacteria and heterotrophic hosts of T4-like phages it is not surprising that the study of cyanomyoviruses has revealed novel and fascinating facets of the phage-host relationship. One of the most interesting features of the marine cyanomyoviruses is their possession of a number of genes that are clearly of host origin such as those involved in photosynthesis, like the psbA gene that encodes a core component of the photosystem II reaction centre. Other host-derived genes encode enzymes involved in carbon metabolism, phosphate acquisition and ppGpp metabolism. The impact of these host-derived genes on phage fitness has still largely to be assessed and represents one of the most important topics in the study of this group of T4-like phages in the laboratory. However, these phages are also of considerable environmental significance by virtue of their impact on key contributors to oceanic primary production and the true extent and nature of this impact has still to be accurately assessed.

Figure 1

Cryoelectron micrographs of purified S-PM2 phage particles. (A) Showing one phage particle in the extended form and one in the contracted form both still have DNA in their heads and (B) Two phage particles with contracted tail sheaths, the particle on the left has ejected its DNA. The lack of collar structure is particularly visible in (B). The diameter of the head is 65 nm. Pictures were taken at the University of Warwick with the kind assistance of Dr Svetla Stoilova-McPhie.

Figure 2

Genome comparison of S-PM2, P-SSM2, P-SSM4, Syn9 and T4 to cyanophage S-RSM4. The outer circle represents the genome of cyanophage S-RSM4. Genes are shaded in blue, with stop and start codon marked by black lines, tRNAs are coloured green. The inner five rings represent the genomes of S-PM2, P-SSM2, P-SSM4, Syn9 and T4 respectively. For each genome all annotated genes were compared to all genes in S-RSM4 using BLASTp and orthologues identified. The nucleotide sequence of identified orthologues were aligned and the percentage sequence identity calculated. The shading of orthologues is proportional to sequence identity, with the darker the shading proportional to higher sequence identity.