Most viral diseases, of man, his livestock, and crops are caused by viruses having RNA genomes. Despite the obvious importance, remarkably little is known about the molecular processes of RNA viral replication. Recombinant DNA techniques have provided procedures for addressing the problem, one of these being the ability to engineer cDNA copies of viral RNAs. Transcription of RNA in vitro from cDNAs of wild-type viral sequences has, in several cases, yielded infectious RNA. Mutagenesis at the cDNA level is relatively facile, and RNA transcripts of such altered clones can serve as specific mutants at the RNA level. Sequences with striking resemblance to the internal promoters of tRNA genes, internal control regions (ICRs), also termed A and B boxes, have recently been observed at the 5' termini of the genomic RNAs of brome mosaic virus (BMV)and other viruses. The 5' terminal location, their partial homology to the recently characterized core sequence f the subgenomic promoter, and the deletion analysis of BMV genomic RNA3 provide strong circumstantial evidence that these sequenced function as promoters of (+) strand replication. The Pi has used wild type and mutant transcripts in combination with unique in vitro replication procedures to define the promoters of genomic (-) strand synthesis and subgenomic (+) strand synthesis. However, the promoters of (+) strand genomic RNA synthesis are as yet undefined. In vivo viral RNA synthesis is usually highly asymmetric with many times more (+) strand RNA being synthesized than (-) strand.n tRNA gene transcription in eukaryotes, dependent on polymerase III and associated transcription factors, is a nuclear process. The apparent homology to promoters of tRNAs suggests involvement of host factors in viral (+) strand synthesis, possibly pol III transcription factors are proposed. Experiments are to define which elements within and surrounding the ICRs of BMV are essential and functional in replication. Furthermore, the role of the ICRs in the asymmetry of replication, and in determining the specific molar ratios of genomic RNAs synthesized is addressed. Identification of the host factor(s) involved in replication is essential in that they are the core of the host-pathogen relationship.
