Temporal regulation of baculovirus RNA: overlapping early and late transcripts

Analysis of the temporal sequence of RNA transcription within the Autographa californica nuclear polyhedrosis virus genome revealed individual transcription units composed of overlapping early or late RNAs, or both. High-resolution S1 nuclease mapping within the 3.0-kilobase HindIII-K fragment located five overlapping RNAs (1.07, 1.38, 2.63, 3.16, and 3.50 kilobases) transcribed in the same direction and terminated at a common 3' site. The smallest RNAs appeared early but were replaced in time by successively larger RNAs initiated further upstream. Primer extension analysis supported the contention that this temporal regulation involved the sequential activation of upstream promoters and the coordinate deactivation of downstream promoters. As such, transcription from upstream genes may suppress that of downstream genes via transcriptional interference (promoter occlusion) and thereby facilitate sequential expression of different viral functions. In contrast, overlapping RNAs with extended 3' ends were transcribed from the abundantly expressed p10 and polyhedrin genes mapping to the HindIII-Q,P/EcoRI-P and HindIII-V/EcoRI-I fragments, respectively. These RNAs were synthesized maximally during the very late occlusion phase and consisted of a major small transcript and several larger but less abundant transcripts.