A new stable and reliable method for labeling nucleic acids of fully replicative viruses.

Efficiently labeling nucleic acids of fully replicative viruses is a challenge. In this work, a 'molecular light switch' complex [Ru(phen)(2)(dppz)](2+), where phen = 1,10-phenanthroline and dppz = dipyrido[3,2-a:2',3'-c]phenazine, has been exploringly used to label vaccinia virus nucleic acid. The labeled virions exhibited strong and stable fluorescence and could be imaged at the single-virion level. Moreover, they were fully infectious and can be used to study the behaviors of invasion into their host cells. The method is general and suitable for labeling various DNA viruses.