Expression kinetics and mapping of the thymidine kinase transcript and an immediate-early transcript from channel catfish virus.

Three transcripts from the terminal repeat of the channel catfish virus (CCV; also known as ictalurid herpesvirus 1) genome were mapped by S1 nuclease and primer extension analyses as well as by cDNA sequencing. These transcripts, TR3, TR5/6, and TR6, are encoded by open reading frame (ORF) 3, ORFs 5 and 6, and ORF 6, respectively, and correspond to those previously identified by sequence analysis (A. J. Davison, Virology 186:9-14, 1992). ORF 5 has previously been determined to encode thymidine kinase, but ORF 3 and ORF 6 encode proteins of unknown function. Although all three transcripts accumulate to high levels in cells infected in the presence of cycloheximide, kinetic analysis demonstrates that TR5/6 and TR6 are either early or late transcripts that leak through the cycloheximide block. In addition, two transcripts from the terminal repeat of the CCV genome that were mapped previously and were thought to be immediate-early in character, TR8a/9 and TR9, exhibit kinetics characteristic of early or late transcripts. TR3 is an immediate-early transcript that appears to have a very short half-life. In the 3' untranslated region of TR3, there are three copies of an AU-rich element which has previously been shown to be involved in destabilization of the oncogene c-fos and granulocyte/macrophage colony-stimulating factor mRNAs. mRNA destabilization may represent another mechanism by which herpesviruses regulate the rapid switch in expression from immediate-early genes to early genes during the transition to the early phase of infection.