Evaluation of MDCK sequences required for RNA pol I promoter activity. As indicated, various lengths of MDCK sequences upstream of the RNA pol I promoter transcription initiation site were cloned into an artificial vRNA-CAT reporter construct. These constructs were individually combined with expression plasmids for PB1, PB2, PA and NP proteins and transfected into MDCK cells. At 44 hrs after transfection, cell lysates were analyzed for CAT expression by a colorimetric ELISA assay. In the plasmid pHW72-CAT, the human RNA pol I promoter directs transcription of a negative sense CAT gene.