Effect of cRNA promoter mutations on temperature sensitive vRNA synthesis. 293T cells were transfected and incubated as described for figure 2, using either a wild type pPol-I(+)NSCAT or two promoter mutants that transplant a vRNA-like base pair into the cRNA promoter; mutants 3'U-8'A (5'-end) or 3G-8C (3'-end). A. Total RNA was harvested and analyzed by primer extension as described before to detect virus-derived CAT RNA species. B. Radiolabelled products for m- and vRNA were quantified by densitometry. Mean ratio of vRNA:mRNA at 37°C and 39°C from two independent experiments is plotted.