Nondisruptive, sequence-specific coupling of fluorochromes to plasmid DNA

A method to attach a fluorochrome sequence-specifically to supercoiled plasmid DNA (pDNA) without perturbing transgene expression would provide an invaluable aid in a variety of applications requiring probes for the intracellular tracking of transfected pDNA. Here we report a method to couple commercially available fluorochromes covalently and sequence-specifically to pDNA using a peptide nucleic acid (PNA) as a linker molecule. The terminal cysteine thiol group on the PNA peptide backbone is reacted with a maleimide moiety on the fluorochrome to produce a fluorescent conjugate which is in turn hybridized to a plasmid expression vector containing an 11-bp target sequence. Spectroscopic evaluation and an electrophoretic mobility shift assay showed that the pDNA hybridized to one PNA–fluorochrome conjugate molecule. The fluorescence signal comigrated with pDNA on acrylamide gels, confirming the stable attachment of the fluorescent conjugate to the pDNA. The utility of one of the conjugates, PNA–Oregon green 488/pCMVβ-DTS, to probe pDNA transport across the nuclear envelope, a significant barrier to gene transfer, was undertaken using a digitonin-permeabilized HeLa cell assay. The PNA–Oregon green 488/pCMVβ-DTS conjugate is able to efficiently traverse the nuclear membrane of the permeabilized cells, accumulating in the nuclei within 30 min and reaching maximal levels by 1 h. When transfected into HeLa cells, the PNA–Oregon green 488/pCMVβ-DTS conjugate retained 55% of the native plasmid’s biological activity, as determined by a β-galactosidase assay. Thus, this method allows for the sequence–specific coupling of commercially available fluorochromes to DNA expression vectors while retaining biological function.