A Regulatory Element Near the 3′ End of the Adeno-Associated Virus rep Gene Inhibits Adenovirus Replication in cis by Means of p40 Promoter-Associated Short Transcripts

Abstract

ABSTRACT Adeno-associated virus (AAV) has long been known to inhibit helper adenovirus (Ad) replication independently of AAV Rep protein expression. More recently, replication of Ad serotype 5 (Ad5)/AAV serotype 2 (AAV-2) hybrid vectors was shown to be inhibited in cis by a sequence near the 3′ end of AAV rep , termed the Rep inhibition sequence for adenoviral replication (RIS-Ad). RIS-Ad functions independently of Rep protein expression. Here we demonstrate that inhibition of adenoviral replication by RIS-Ad requires an active AAV p40 promoter and the 5′ half of the intron. In addition, Ad inhibition is critically dependent on the integrity of the p40 transcription start site (TSS) leading to short p40-associated transcripts. These do not give rise to effector molecules capable of inhibiting adenoviral replication in trans , like small polypeptides or microRNAs. Our data point to an inhibitory mechanism in which RNA polymerase II (Pol II) pauses directly downstream of the p40 promoter, leading to interference of the stalled Pol II transcription complex with the adenoviral replication machinery. Whereas inhibition by RIS-Ad is mediated exclusively in cis , it can be overcome by providing a replication-competent adenoviral genome in trans . Moreover, the inhibitory effect of RIS-Ad is not limited to AAV-2 but could also be shown for the corresponding regions of other AAV serotypes, including AAV-5. These findings have important implications for the future generation of Ad5/AAV hybrid vectors. IMPORTANCE Insertion of sequences from the 3′ part of the rep gene of adeno-associated virus (AAV) into the genome of its helper adenovirus strongly reduces adenoviral genome replication. We could show that this inhibition is mediated exclusively in cis without the involvement of trans -acting regulatory RNAs or polypeptides but nevertheless requires an active AAV-2 p40 promoter and p40-associated short transcripts. Our results suggest a novel inhibitory mechanism that has so far not been described for AAV and that involves stalled RNA polymerase II complexes and their interference with adenoviral DNA replication. Such a mechanism would have important implications both for the generation of adenoviral vectors expressing the AAV rep and cap genes and for the regulation of AAV gene expression in the absence and presence of helper virus.