Expression of reporter genes from the defective RNA CD-61 of the coronavirus infectious bronchitis virus

The defective RNA (D-RNA) CD-61, derived from the Beaudette strain of the a vian coronavirus infectious bronchitis virus (IBV), was used as an RNA vect or for the expression of two reporter genes, luciferase and chloramphenicol acetyltransferase (CAT). D-RNAs expressing the CAT gene were demonstrated to be capable of producing CAT protein in a helper-dependent expression sys tem to about 1.6 mu g per 10(6) cells. The reporter genes were expressed fr om two different sites within the CD-61 sequence and expression was not aff ected by interruption of the CD-61-specific ORF. Expression of the reporter genes was under the control of a transcription-associated sequence (TAS) d erived from the Beaudette gene 5, normally used for the transcription of IB V subgenomic mRNA 5. The Beaudette gene 5 TAS is composed of two tandem rep eats of the IBV canonical consensus sequence involved in the acquisition of a leader sequence during the discontinuous transcription of IBV subgenomic mRNAs. It is demonstrated that only one canonical sequence is required for expression of mRNA 5 or for the expression of an mRNA from a D-RNA and tha t either sequence can function as an acceptor site for acquisition of the l eader sequence.