RESUMEN
The genome of human cytomegalovirus (HCMV) has been studied extensively in some regions, but not others. In this study, transcripts of the genome were further characterised for open reading frames (ORFs) TRL7, UL36, UL65, UL94, US3 and US34, and for the previously unrecognised ORF, UL20a. Reverse transcription-PCR demonstrated the presence of spliced transcripts from the putative glycoprotein gene, UL20a, at early and late times post-infection. US3 full-length and spliced transcripts, including a previously unidentified transcript (US3ii), were described at immediate early times. Sequencing of the complete ORFs of UL20a and US3 from 21 clinical isolates showed that US3 is well conserved in all isolates (97-100% identity), whereas UL20a shows more variation at the nucleotide level, with 90-100% identity. The limits of transcription, and splice donor and acceptor sequences for UL20a and US3 were conserved in all isolates, indicating likely conservation of mRNA splicing patterns. Sequencing a late cDNA library identified the limits of transcription for ORFs TRL7, UL94 and US34 and transcription from the TRL7 ORF was confirmed by northern blotting. Transcripts were found that were congruent with UL36 and UL65, but these differed in the limits previously predicted for these ORFs. These findings show the variation between predicted and actual transcription and indicate the complex nature of transcription from HCMV ORFs.