Messenger RNAs of a strongly‐expressed late gene of cowpox virus contain 5′‐terminal poly(A) sequences.

We have identified and characterized one of the most strongly‐expressed genes of cowpox virus (CPV). This is the gene encoding the major protein component of the A‐type inclusion bodies produced by this virus. This gene (designated the 160K gene) is transcribed late during the infection. Analyses of its mRNAs showed that these late RNAs, unlike all other characterized late mRNAs of poxviruses, are uniform in length. However, the most remarkable feature of the mRNAs of the 160K gene is the structure of their 5′‐termini. Most of these mRNAs have 5′‐terminal poly(A) sequences containing 5‐21 residues. Furthermore, these 5′‐terminal poly(A) sequences are not complementary to the corresponding region of the template strand of the viral DNA. Instead, the nucleotide sequences of the mRNA and the viral DNA diverge at the site of the three As in the sequence 5′‐TAAATG‐3′ containing the gene's initiation codon. Consequently, the poly(A) provides the leader sequences of these mRNAs. These unusual 5′‐terminal structures suggest that the late mRNAs of pox‐virus genes are generated by a novel process.