Intracellular forms of the parental human cytomegalovirus genome at early stages of the infective process

Abstract

Intracellular forms of human cytomegalovirus (HCMV) DNA molecules isolated from infected cells were examined by electron microscopy before and after the onset of viral DNA synthesis. In cells harvested before showing replicative forms of viral DNA, circular and concatemeric molecules were observed in addition to linear double-stranded molecules. The observation of circular, unbranched, unit-size molecules suggests that HCMV DNA has a repetitive sequence that is located at or near the termini and is exposed by exonuclease digestion within infected cells, and that single-stranded regions can complement each other to form circles. Linear molecules (unit size or smaller) and concatemers with replicative “eye” loops or forks were observed after DNA replication could be assumed to have begun. Viral DNA molecules with terminal loops were also observed. These structures indicate that an inverted repetition of the sequence may be present within the terminal region. The functions of these molecules and of circular molecules are unknown.