The genomic RNA ofpotyviruses has a characteristic 5' non-translated region (5'NTR) to which a viral protein, VPg, is covalently attached. This suggests that the viral RNA lacks a conventional cap structure and thus its translation may not proceed in the same way as most cellular mRNAs. To investigate the role of the 5'NTR during translation, various derivatives of the turnip mosaic potyvirus (TuMV) leader were fused to the reporter gene fl-glucuronidase (GUS). These constructs were used to monitor the efficiency of translation in vitro in a rabbit reticulocyte lysate and in planta following microprojectile DNA delivery into tobacco cell suspensions. GUS transcripts fused with the TuMV 5'NTR, whether they were capped or not, were efficiently translated, whereas GUS transcripts without the viral leader needed to be capped for expression. When transcripts of the viral leader were supplied in excess over functional transcripts, translation was inhibited in a dose-dependent manner. Similarly, transcripts synthesized from the reverse complement of the 5'NTR inhibited translation to the same extent as the wild-type sequence, indicating that cap independence was not conferred by a specific sequence within the viral leader. A stable hairpin loop was placed in front or after the viral sequence. This hairpin loop normally prevented translation of control GUS transcripts but when the viral leader was positioned after it a significant level of GUS activity was measured, whether the transcripts were capped or not. On the other hand, when the hairpin loop was positioned after the viral leader, no GUS activity was measured. These results suggested that ribosomes bound to an internal site within the TuMV 5'NTR and then presumably scanned the sequence for the initiator AUG.
The expression kinetics of an essential trans-regulatory protein, ICP27, from herpes simplex virus type 1 correspond to that of an immediate-early gene whose expression increases rapidly upon infection and then decreases as of 7 hr postinfection. In contrast, here we report that the bovine herpesvirus type 1 (BHV-1) homolog BICP27, a 50-kDa protein, is expressed as an early gene. Both the transcript and protein accumulated gradually reaching peak levels at approximately 12 hr postinfection, after which point steady state levels were maintained up to 24 hr. Thus the expression profiles of ICP27 and BICP27 are significantly different, suggesting that they may possess different functions.
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