FIG. 1.

The UL24 transcription unit. (A) The top line represents the prototype organization of the HSV-1 genome; the shaded boxes represent the repeat segments, and the thin lines represent the unique regions. The UL24 gene lies at 0.3 map unit. Below is an expanded view of the locus containing UL24. The UL24 ORF is shown as a filled box with an arrow indicating the direction of transcription. The tk and UL25-UL26.5 ORFs are shown as open boxes with arrows. The six UL24 transcripts (thin lines with arrows) originate from three different transcription start sites and terminate at either the UL24 or the UL26 polyA signal. The transcripts that arise from the first two start sites (5.6, 5.4, 1.4, and 1.2 kb) contain the entire UL24 ORF. The other two transcripts (5.2 and 0.9 kb) originate from a start site within the ORF. The three transcripts that utilize the UL24 polyA signal (1.4, 1.2, and 0.9 kb) exhibit early kinetics (β), while the three that terminate at the UL26 signal (5.6, 5.4, and 5.2 kb) exhibit leaky-late kinetics (γ1). These longer transcripts contain sequences corresponding to the UL24, UL25, UL26 and UL26.5 genes. Transcripts arising from the second start site are the least abundant, as indicated by the thinner lines. The UL24 and tk genes are transcribed divergently, and the 5′ ends of the genes overlap. (B) 5′ UTR of UL24. The sequence from HSV-1 strain KOS of the region upstream of the UL24 ORF is shown with the ATG for the initiation codon in bold (19), with the same numbering system as that used by McGeoch et al. (26). Arrows above the sequence indicate the two transcription start sites for UL24 mRNAs that could encode a full-length protein. The short upstream ORF is boxed. This feature is also present in HSV-1 strain 17 and HSV-2 strains G and HG52.