Expression of unspliced NS₁ mRNA, spliced NS₂ mRNA, and a spliced chimera mRNA from cloned influenza virus NS DNA in an SV40 vector

Influenza virus gene 8 codes for two nonstructural proteins (NS₁ and NS₂) which are translated, respectively, from a colinear and an interrupted mRNA. To investigate the mechanism of transcription processing by splicing, cloned full-length NS DNA was inserted into the late region of an SV40 vector. In this manner, transcription of NS RNA from the recombinant initiates and terminates using SV40 control sequences. RNA mapping and sequence analysis showed that the RNA transcripts are processed to produce both the spliced and unspliced NS mRNAs in approximately equal abundance. The junction sequence of the spliced mRNA is identical to that of NS₂ mRNA found in influenza virus-infected cells. In addition, another mRNA species was found to contain a chimera splice between SV40 and NS sequences. Both NS₁ and NS₂ polypeptides are produced in the recombinant-infected cells as predicted from sequence analysis. These studies establish that during influenza virus infections processing of the NS, mRNA transcript undergoes a mechanism of splicing similar to that occurring with DNA-directed RNA transcription.