Proof of ssDNA degraded from dsDNA for ET recombination

The widely used ET recombination requires an ssDNA product degraded by Rac phage protein E588 from dsDNA for strand invasion. However, proof of the ssDNA product is still elusive. The study provided three levels of proof sequentially. The probable ssDNAs degraded by E588 from the fluorescent plus-, minus-, or double-stranded dsDNA pET28a-xylanase exhibited a half fluorescence intensity of the corresponding dsDNAs, equivalent to the E588 degradation nucleotides half that of the total nucleotides degraded from the corresponding dsDNA. The ssDNA product degraded by E588 from the fluorescent minus-stranded dsDNA was confirmed by gradient gel-electrophoresis and two nuclease degradation reactions. Degraded by E588 from the dsDNA pET28a-xylanase that had a phosphorothioated plus-stranded 5′-terminus, the plus-stranded ssDNA product was separated via gel electrophoresis and recovered via a DNAclean kit. The recovered ssDNA product was proven to have intact 5′- and 3′-ends by DNA sequencing analysis. This study provides a solid foundation for the mechanism of ssDNA invasion.