Complete genome assembly and methylome dissection of PL15/H .

Although restriction-modification systems are found in both Eubacterial and Archaeal kingdoms, comparatively less is known about patterns of DNA methylation and genome defense systems in archaea. Here we report the complete closed genome sequence and methylome analysis of PL15/H , a strain of the CO-reducing methanogenic archaeon and a commercial source for I, II, and III restriction endonucleases. The PL15/H genome consists of a 1.68 megabase circular chromosome predicted to contain 1,615 protein coding genes and 38 tRNAs. A combination of methylome sequencing, homology-based genome annotation, and recombinant gene expression identified five restriction-modification systems encoded by this organism, including the methyltransferase and site-specific endonuclease of III. The III restriction endonuclease was recombinantly expressed, purified and shown to have site-specific DNA cleavage activity .