We next examined the protein-binding properties of hnRNP A1DC. Since hnRNP A1 has been shown to interact with the N protein, which also participates in MHV RNA synthesis (Compton et al., 1987; Wang and Zhang, 1999) , we ®rst determined whether the dominantnegative mutant of hnRNP A1 retained the ability to interact with the N protein in vitro. GST pull-down assay using various truncation mutants of hnRNP A1 showed that the N protein bound the N-terminal domain (aa 1±163) of hnRNP A1 ( Figure 7B) ; thus, the binding of hnRNP A1DC [equivalent to hnRNP A1(1±245)] to the N protein was not affected. We next examined the in vivo interaction of the wt and mutant hnRNP A1 with an MHV ORF 1a product, p22, which has been shown to co-localize with the de novo synthesized viral RNA (S.T.Shi and The viruses were passaged twice in wt DBT cells to obtain P1 and P2 viruses. Cytoplasmic RNA was extracted from the DBT cells infected with P0, P1 and P2 viruses and treated with glyoxal before electrophoresis and northern blot analysis using a 32 P-labeled (±)-strand mRNA 7 as a probe. The A59 DI RNA and DIssE RNA are indicated by arrows.