HOST-DERIVED 5' ENDS AND OVERLAPPING COMPLEMENTARY 3' ENDS OF THE 2 MESSENGER-RNAS TRANSCRIBED FROM THE AMBISENSE-S SEGMENT OF UUKUNIEMI VIRUS

Two mRNAs, coding for the N and NS(S) proteins, are transcribed from the small (S) Uukuniemi virus RNA segment by an ambisense strategy (J. F. Simons, U. Hellman, and R. F. Petterson, J. Virol. 64:247-255, 1990). In this report, we describe the analysis of the 5' and 3' ends of the two mRNAs. Primer extension as well as cloning and sequencing of individual mRNAs showed that the 5' ends of both mRNAs contained nonviral sequences ranging from 7 to 25 residues in length (mean, 12 residues), indicating a cap-snatching mechanism similar to the one originally described for priming of influenza virus mRNA synthesis. In 35% of the cases, the first virion-specified nucleotide (an A residue) was substituted with a G residue. Between the translation termination codons of N and NS(S), there is a 74-residue-long noncoding intergenic region (Simons et al., J. Virol. 64:247-255, 1990). Nuclease protection assays using both RNA and DNA hybridization probes showed that the 3' ends of the N and NS(S) mRNAs overlap each other by about 100 nucleotides. The 3' end of the NS(S) mRNA extends into the coding sequence of the N mRNA, whereas the N mRNA is terminated just prior to the stop codon of NS(S). To our knowledge, this is the first example of overlapping complementary mRNAs in viruses with an ambisense coding strategy. No obvious transcription termination sequence was identified. However, because of a short palindromic sequence in the intergenic region, the 3' ends of both mRNAs (and consequently also the template RNAs) can be folded into an A/U-rich hairpin structure. It remains to be determined whether this structure plays any role in transcription termination.