Bacteriophages are considered to be the most abundant biological entities on the planet. The Siphoviridae are the most commonly encountered tailed phages and contain double-stranded DNA with an average genome size of ϳ50 kb. This paper describes the isolation from four different activated sludge plants of the phage RRH1, which is polyvalent, lysing five Rhodococcus species. It has a capsid diameter of only ϳ43 nm. Whole-genome sequencing of RRH1 revealed a novel circularly permuted DNA sequence (14,270 bp) carrying 20 putative open reading frames. The genome has a modular arrangement, as reported for those of most Siphoviridae phages, but appears to encode only structural proteins and carry a single lysis gene. All genes are transcribed in the same direction. RRH1 has the smallest genome yet of any described functional Siphoviridae phage. We demonstrate that lytic phage can be recovered from transforming naked DNA into its host bacterium, thus making it a potentially useful model for studying gene function in phages.
There are thought to be at least 10 31 phage particles on the planet (22), making them the most abundant of biological entities. Our understanding of their genetic variety is scant, and we have only just begun to explore this enormous phage diversity (35). Phages are classified on the basis of their morphology into 13 families and one named order (Caudovirales) that embraces more than 95% of all described phages (3). The Caudovirales are divided into three families differing in their tail morphology and function: the Myoviridae (contractile tails), the Podoviridae (short noncontractile tails), and the Siphoviridae (long noncontractile tails) (2). The most commonly observed phages (ϳ60%) are members of the Siphoviridae (1).Siphoviridae phages contain typically 35 to 70 kb of DNA in either an icosahedral capsid ranging in size from 55 to 60 nm or a prolate capsid (2,20). Despite phage lambda, a member of this family, being one of the most studied model organisms, what constitutes the core genome of this family is still unknown. Complicating any attempt to determine the minimal functional Siphoviridae genome is that the known genomes cover a wide size range (16.9 kb to more than 121 kb), a lack of gene conservation between phages (8), and DNA packing constraints limit the ability to create deletion mutants (10). Identification of new phages with inherently small genomes and gene sets should provide useful models for unraveling the complexities of phage reproduction and ecology (39).One group of phages that has received little attention is the group of phages that target Rhodococcus species, organisms found in a wide range of habitats, especially soil (16). Most species are nonpathogenic, although R. equi causes serious infections in foals (28), while other strains are responsible for operational problems in wastewater treatment systems (30). Summers et al. (41) and Petrovski et al. (32) have characterized five phages able to propagate on Rhodococcus species. These phages all have novel genome sequence...