SPLICING OF CAULIFLOWER MOSAIC-VIRUS 35S RNA IS ESSENTIAL FOR VIRAL INFECTIVITY

A splicing event essential for the infectivity of a plant pararetrovir us has been characterized. Transient expression experiments using repo rter constructs revealed a splice donor site in the leader sequence of the cauliflower mosaic virus (CaMV) 35S RNA and three additional spli ce donor sites within open reading frame (ORF) I. All four donors use the same splice acceptor,within ORF II. Splicing between the leader an d ORF II produces an mRNA from which ORF III and, in the presence of t he CaMV translational transactivator, ORF IV can be translated efficie ntly. The other three splicing events produce RNAs encoding ORF I-II i n-frame fusions. All four spliced CaMV RNAs were detected in CaMV-infe cted plants. Virus mutants in which the splice acceptor site in ORF II is inactivated are not infectious, indicating that splicing plays an essential role in the CaMV life cycle. The results presented here sugg est a model for viral gene expression in which RNA splicing is require d to provide appropriate substrate mRNAs for the specialized translati on mechanisms of CaMV.