Mapping early transcripts of herpes simplex virus type 1 by electron microscopy.

RNA displacement loop patterns in intact herpes simplex virus DNA and herpes simplex virus DNA restriction fragments indicate that viral RNA associated with polyribosomes early after infection is transcribed from three major areas of the genome. One area of early transcription is in the short segment of the viral DNA and is roughly delineated by the inverted repeat sequences bounding this segment. The other two areas of early mRNA transcription map in the long segment. Each of three major areas of early mRNA transcription can be further resolved into several regions of freqent looping bordered by regions in which RNA displacement loops are rare. These regions range in size from about 1.5 kilobases to about 9 kilobases with a mean size of about 3.5 kilobases. Although the data do not allow precise assignment of individual early gene locations, it is seen, even at the lowest level of resolution, that the early genes are not completely contiguous but are distributed along the length of the herpes simplex type 1 viral genome.