Isolation and genomic analysis of a type IV pili-independent Thermus thermophilus phage, OMN1 from a Japanese hot spring

A Thermus thermophilus lytic phage was isolated from a Japanese hot spring using a type IV pili-deficient strain as an indicator host, and designated as OMN1. Electron microscopic (EM) examination revealed that OMN1 had an icosahedral head and a contractile tail, suggesting that OMN1 belonged to Myoviridae. An EM analysis focused on OMN1 adsorption to the Thermus host cell showed that the receptor molecules for the phage were uniformly distributed on the outer surface of the cells. The circular double-stranded DNA of OMN1 was 76,659 base pairs in length, and the guanine and cytosine content was 61.8%. It was predicted to contain 99 open reading frames, and its putative distal tail fiber protein, which is essential for non-piliated host cell surface receptor recognition, was dissimilar in terms of sequence and length with its counterpart in the type IV pili-dependent OYS40. A phage proteomic tree revealed that OMN1 and OYS40 are in the same cluster, but many genes had low sequence similar-ities and some seemed to be derived from both mesophilic and thermophilic organisms. The gene organization suggested that OMN1 evolved from a non-Thermus phage through large-scale recombi-nation events of the genes determining the host specificity, followed by gradual evolution by recom- bination of both the thermophilic and mesophilic DNAs assimilated by the host Thermus cells. This newly isolated phage will provide evolutionary insights into thermophilic phages.