Abstract
Shiga toxin 2 (Stx2), one of the most important virulence factors of enterohaemorrhagic Escherichia coli (EHEC), is encoded by phages. These phages (Stx2 phages) are often called lambda-like. However, most Stx2 phages are short-tailed, thus belonging to the family Podoviridae, and the functions of many genes, especially those in the late region, are unknown. In this study, we performed a systematic genetic and morphological analysis of genes with unknown functions in Sp5, the Stx2 phage from EHEC O157:H7 strain Sakai. We identified nine essential genes, which, together with the terminase genes, determine Sp5 morphogenesis. Four of these genes most likely encoded portal, major capsid, scaffolding and tail fiber proteins. Although exact roles/functions of the other five genes are unknown, one was involved in head formation and four were required for tail formation. One of the four tail genes encoded an unusually large protein of 2,793 amino-acid residues. Two genes that are likely required to maintain the lysogenic state were also identified. Because the late regions of Stx2 phages from various origins are highly conserved, the present study provides an important basis for better understanding the biology of this unique and medically important group of bacteriophages.
Highlights
Shiga toxin 2 (Stx2), one of the most important virulence factors of enterohaemorrhagic Escherichia coli (EHEC), is encoded by phages
Studies have shown that bacteriophages, those that can be integrated into the host chromosomes, play more critical roles than was previously recognized in the evolution of a wide range of bacteria by carrying various genes in host cells[1,2,3]
While a high level of Stx2a is produced upon phage induction, the level of Stx2c produced by EHEC O157 is generally very low, even after phage induction, and the expression of Stx[1] production is iron-regulated and not linked to phage induction[18,19,20,21,22]
Summary
Shiga toxin 2 (Stx2), one of the most important virulence factors of enterohaemorrhagic Escherichia coli (EHEC), is encoded by phages. We performed a systematic genetic and morphological analysis of genes with unknown functions in Sp5, the Stx[2] phage from EHEC O157:H7 strain Sakai. Studies have shown that bacteriophages, those that can be integrated into the host chromosomes (called temperate phages), play more critical roles than was previously recognized in the evolution of a wide range of bacteria by carrying various genes in host cells[1,2,3] This expanding sequence information about phages/prophages has identified a huge number of phage-encoded genes with unknown functions[4,5]. We describe the results of a systematic genetic and morphological analysis of the genes of unknown function encoded by a representative Stx2a phage, Sp5 of O157:H7 strain Sakai[32,36]. The results of electron microscopic examinations, including observation using an immunogold labeling technique, are presented to assign the proteins’ functions in Sp5 morphogenesis
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