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The nucleotide sequence determination of the genome of the Beaudette strain of the coronavirus avian infectious bronchitis virus (IBV) has been completed. The complete sequence has been obtained from 17 overlapping cDNA clones, the 5'-most of which contains the leader sequence (as determined by direct sequencing of the genome) and the 3'-most of which contains the poly(A) tail. Approximately 8 kilobases at the 3' end of this sequence have already been published. These contain the sequences of mRNAs A to E within which are the genes for the spike, the membrane and the nucleocapsid polypeptides: the main structural components of the virion. The remainder of the sequence, equivalent to the 'unique' region of mRNA F, is some 20 kilobases in length and is thought to code for a polymerase or polymerases which are involved in the replication of the genome and the production of the subgenomic messenger RNAs. This sequence contains two large open reading frames, potentially coding for polypeptides of molecular weights 441,000 and 300,000. Unlike other large open reading frames in the virus, the 300,000 open reading frame appears to have no subgenomic RNA associated with it which would allow it to be at the 5' end of an mRNA species. Because of this, and because of the characteristics of the sequence in the region immediately upstream of its start codon, other mechanisms of translation, such as ribosome slippage, must be postulated.
Boursnell et al. (Thu,) studied this question.