THE COMPLETE NUCLEOTIDE SEQUENCE OF AVIAN INFECTIOUS BRONCHITIS VIRUS: ANALYSIS OF THE POLYMERASE-CODING REGION INTRODUCTION M. E. G. Boursnell, T. D. K. Brown, I. J. Foulds, P. F. Green, F. M. Tomley and M. M. Binns Houghton Poultry Research Station Houghton Huntingdon Cambridgeshire PE17 2DA England, UK Avian infectious bronchitis virus (IBV) is the type species of the family Coronaviridae (Siddell et al., 1983). It has a large positive-stranded RNA genome which has been estimated at 20-24 kilobases (Lomniczi & Kennedy, 1977). As with other coronaviruses, a number of subgenomic messenger RNA species are produced in infected cells which form a 3'-coterminal nested set (Stern & Kennedy, 1980a; 1980b). In the case of IBV there are six mRNA species in total, which are designated mRNAs A-F, mRNA A being the smallest, and mRNA F being of genome size. mRNAs A, C and E encode the three main structural components of the virion, the nucleocapsid polypeptide, the membrane polypeptide and the precursor polypeptide to the spike (Stern & Sefton, 1984). mRNA D encodes at least one product, a 12.4 kilodalton polypeptide of unknown function (Smith et al., this volume), but no product has yet been detected for mRNA B. The coding regions of mRNAs A-E are situated in the 3'-most 7.3 kilobases of the IBV genome, the nucleotide sequence of which has been determined previously from cDNA clones (Boursnell et al., 1984, 1985a, 1985b; Boursnell & Brown, 1984; Binns et al., 1985b). RNase T fingerprint analysis reveals no difference between messenger RNA F, the genome-sized mRNA present in infected cells, and the virion RNA (Stern & Kennedy, 1980a), although the possibility of minor differences between them cannot be ruled out. If they are taken as being identical, then the remainder of the IBV genome constitutes the 'unique' region of mRNA F, in other words that part of mRNA F not present in the smaller mRNAs. Because the genome is infectious (Lomniczi, 1977) and because there is no evidence for a virion-associated RNA polymerase (Schochetman et al., 1977) this region of the genome is thought to encode a polymerase or polymerases which carry out the necessary replication and transcription functions of the virus. We have now determined the nucleotide sequence, from cDNA clones, of the 'unique' region of mRNA F. This completes the sequence of the IBV genome. 15 M. M. C. Lai et al. (eds.), Coronaviruses © Plenum Press, New York 1987