Abstract
This chapter discusses some observations concerning the natural occurrence and structural organization of polycistronic animal virus mRNAs, and the mechanisms by which they may be translated to yield two or more unique polypeptide products. In most polycistronic viral mRNAs, initiation of translation of both the 5’-proximal, upstream cistron and the internal, downstream cistron(s) likewise occurs at an AUG codon. Animal viruses encoding polycistronic mRNAs in which translation-initiation occurs alternatively at one or more AUG initiation sites, include members of several virus families that utilize a variety of different replication strategies as parts of their life cycles. They include: 1. viruses with DNA genomes and viruses with RNA genomes; 2. viruses with circular genomes and viruses with linear genomes; 3. viruses whose genomes are constituted by a single piece of nucleic acid, as well as viruses with segmented genomes; and 4. viruses that utilize the cell nucleus as the site for mRNA biogenesis, as well as viruses whose mRNA is synthesized in the cytoplasm. Furthermore, many different biochemical mechanisms may exist in animal cells to permit the expression of functionally polycistronic viral mRNAs.