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 viru s (CCV; also known as ictalurid herpesvirus 1) genome were mapped by S 1 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, ORPs 5 and 6, and ORF 6, respectively, and correspond t o those previously identified by sequence analysis (A. J. Davison, Vir ology 186:9-14, 1992). ORF 5 has previously been determined to encode thymidine kinase, but ORF 3 and ORF 6 encode proteins of unknown funct ion. Although all three transcripts accumulate to high levels in cells infected in the presence of cycloheximide, kinetic analysis demonstra tes 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 k inetics characteristic of early or late transcripts. TR3 is an immedia te-early transcript that appears to have a very short half-life. In th e 3' untranslated region of TR3, there are three copies of an AU-rich element which has previously been shown to be involved in destabilizat ion of the oncogene c-fos and granulocyte/macrophage colony-stimulatin g factor mRNAs, mRNA destabilization may represent another mechanism b y which herpesviruses regulate the rapid switch in expression from imm ediate-early genes to early genes during the transition to the early p hase of infection.