Abstract
The T5-like siphoviruses DT57C and DT571/2, isolated from horse feces, are very closely related to each other, and most of their structural proteins are also nearly identical to T5 phage. Their LTFs (L-shaped tail fibers), however, are composed of two proteins, LtfA and LtfB, instead of the single Ltf of bacteriophage T5. In silico and mutant analysis suggests a possible branched structure of DT57C and DT571/2 LTFs, where the LtfB protein is connected to the phage tail via the LtfA protein and with both proteins carrying receptor recognition domains. Such adhesin arrangement has not been previously recognized in siphoviruses. The LtfA proteins of our phages are found to recognize different host O-antigen types: E. coli O22-like for DT57C phage and E. coli O87 for DT571/2. LtfB proteins are identical in both phages and recognize another host receptor, most probably lipopolysaccharide (LPS) of E. coli O81 type. In these two bacteriophages, LTF function is essential to penetrate the shield of the host’s O-antigens. We also demonstrate that LTF-mediated adsorption becomes superfluous when the non-specific cell protection by O-antigen is missing, allowing the phages to bind directly to their common secondary receptor, the outer membrane protein BtuB. The LTF independent adsorption was also demonstrated on an O22-like host mutant missing O-antigen O-acetylation, thus showing the biological value of this O-antigen modification for cell protection against phages.
Highlights
Bacteriophage T5, infecting many laboratory strains of E. coli, is one of the series of seven “authorized” “T” phages selected by Max Delbrück that have served as model organismsViruses 2016, 8, 26; doi:10.3390/v8010026 www.mdpi.com/journal/virusesViruses 2016, 8, 26 for basic research in phage biology since the 1940s [1]
T5-like phages are considered a genus in the Siphoviridae family with at least 11 representatives with fully sequenced genomes annotated in GenBank as “T5likeviruses”: coliphages T5, BF23, CEV2, H8, and bv_EcoS_AKF33; Salmonella phages SPC35, EPS7, and Stitch; Yersinia phage phiR201; Pectobacterium phage My1, and Vibrio phage pVp1 [2,3,4,5,6,7]
In the course of our ecological project performed in 2006, a series of five bacteriophages closely related to coliphage T5 were isolated from a single sample of the horse feces on lawns of the laboratory
Summary
Bacteriophage T5, infecting many laboratory strains of E. coli, is one of the series of seven “authorized” “T” phages selected by Max Delbrück that have served as model organisms. Viruses 2016, 8, 26 for basic research in phage biology since the 1940s [1]. T5-like coliphages are widespread in nature, including as components of human and animal intestinal microbiomes [10,11], see [12,13]. They are found in downstream habitats such as sewage or soils. The analysis of the multiple, closely related isolates recovered from related environmental samples may shed light on the mechanisms of short-term adaptation of the phage genome
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