Characterization of Streptomyces plasmid-phage pFP4 and its evolutionary implications

Autonomous-replicating plasmid pFP4 of Streptomyces sp. FR1 isolated from a heavy metal-contaminated land was cloned and sequenced. Surprisingly, the 40,949-bp pFP4 contains a cluster of 20 genes, resembling these chromosome-integrated prophages of Streptomyces sp. SPB78 and Streptomyces scabiei 87.22. Plasmid pFP4 could transfer by conjugation and a replication locus, iteron/repA/repB, was identified. The filtered FR1 culture could infect both FR1 and FR1 cured of pFP4 to form plaques, and also six out of 13 strains from the same land, but failed to form plaques on other seven strains from same source and all ten Streptomyces species from different sources. pFP4 phage particles were observed by transmission electron microscopy. Major structural proteins (capsid, portal and tail, etc.) of pFP4 virions were encoded by twelve pFP4 genes. pFP4 phage DNA contained 3′ protruding cohesive ends of 9-nt. Streptomyces pFP4 represents a novel plasmid-phage.

► We sequenced an autonomous-replicating plasmid pFP4 of Streptomyces sp. FR1. ► A cluster of 20 genes on pFP4 resembled these prophages of Streptomyces. ► A replication locus of pFP4 was identified. ► The filtered FR1 culture could infect both FR1 and FR1 cured of pFP4. ► pFP4 phage structural proteins and the cohesive ends of its DNA were identified.