Single-Stranded Regions in the Genome of the Ectocarpus siliculosus Virus

The double-stranded DNA genome of the Ectocarpus siliculosus virus, EsV, is interrupted by numerous single-stranded gaps. We have investigated whether single-stranded regions occur at random or at specific sites. A brief treatment with a single-strand-specific endonuclease dissected the genome into two large fragments, but more extensive treatment produced a spectrum of nuclease-resistant fragments, mainly 10 to 70 kb in size. Native EsV DNA was also used as a substrate for gap-filling DNA synthesis. Restriction analysis revealed that essentially all of the 60 or more SacI restriction fragments became labeled but a few fragments were more intensely labeled than others. The EsV genome may contain a few extended single-stranded regions at fixed sites in addition to numerous single-stranded regions probably occurring at random and varying sites.