THE DNA-SEQUENCE CODING FOR THE 5' UNTRANSLATED REGION OF HERPES-SIMPLEX VIRUS TYPE-1 ICP22 MESSENGER-RNA MEDIATES A HIGH-LEVEL OF GENE-EXPRESSION

The sequence coding for the 5' untranslated region (UTR) of ICP22 mRNA of herpes simplex virus type 1 has been tested for its ability to reg ulate gene expression. This sequence was placed in frame with the chlo ramphenicol acetyltransferase (CAT) coding sequence and under the cont rol of the simian virus 40 early promoter-enhancer. Under these condit ions, the sequence coding for the 5'UTR led to an increase of about 13 -fold in CAT activity, measured during transient expression. The use o f mutants with progressive deletions within the sequence coding for th e 5'UTR allowed localization ofthe sequence responsible for the enhanc ement of gene expression to the first exon of the ICP22 gene. Precise quantification of hybrid ICP22-CAT mRNA showed that the sequence codin g for the 5'UTR induced an increase in the amounts of transcripts, whi ch resulted in a parallel increase in CAT activity. This increase in t he level of hybrid ICP22-CAT mRNA is not the result of an increase in mRNA stability, nor is it due to more efficient nucleo-cytoplasmic tra nsport of the transcripts. Moreover, the distribution of hybrid mRNA i n the different ribosomal populations indicates that the 5'UTR of ICP2 2 mRNA does not induce a preferential recruitment of the transcripts b y the translational apparatus. Taken together, these results indicate that a cis-acting element located in the sequence coding for the 5'UTR of ICP22 mRNA can mediate a high level of gene expression independent ly of the viral promoter and of viral trans-acting factors.