Multiple spliced and unspliced transcripts from human cytomegalovirus immediate-early region 2 and evidence for a common initiation site within immediate-early region 1

Human cytomegalovirus immediate-early (IE) region 2 (0.732 to 0.740 map unit) begins 35 nucleotides downstream of IE region 1 (Stenberg et al., J. Virol. 49:190-199, 1984). A series of mRNAs that have different splicing patterns are transcribed from region 2. There is an unspliced 1,589-nucleotide exon present in minor amounts and two spliced exons (836 and 289 nucleotides) present in larger amounts. The IE region 2 exons were found to be spliced onto the first three exons of region 1. Therefore, under IE conditions the region 1 promoter-regulatory region can promote transcription of region 2. Promoter sequences (i.e., CAAT and TATA boxes) are found upstream of the 5' end of IE region 2 but presumably function poorly at IE times after infection. The transcriptional regulation of these IE genes and the possible functional roles of the proteins are discussed. We postulate that a series of unique but related proteins are made from the region 2 transcripts. Some of these proteins should contain the same 169 amino-terminal residues as the major IE 72-kilodalton protein encoded by IE region 1 (Stenberg et al., J. Virol. 49:190-199, 1984). Variations in the amino acid sequences of the region 2 proteins could occur at either the amino terminus, the carboxy terminus, or both termini.