CHARACTERIZATION OF NUCLEAR FACTORS INVOLVED IN 202-GENE INDUCTION BYIFN-ALPHA, VIRUSES OR DSRNA IN MURINE LEUKEMIA-CELLS

When treated with IFN-alpha, L1210 leukemia cells express high levels of the mouse 202 gene mRNA after a few hours. Three tandem copies of a 43 bp fragment (GAbox) homologous to the IFN-stimulatable response el ement (ISRE), located in the 5'-flanking region of the 202 gene, were linked to the reporter CAT gene and transiently transfected into L1210 cells. The data suggest that the GA box is sufficient to confer trans criptional inducibility upon IFN stimulation. Binding assays, using th e labeled GA box as a probe, demonstrated the presence of a retarded c omplex, designated GAbfl, in the nuclear extracts of L1210 cells treat ed with IFN-alpha. This complex is absent in the extracts of L1210 cel ls treated with ssRNA viruses or synthetic dsRNA. Moreover, photoaffin ity crosslinking experiments revealed that GAbfl contains a protein of about 50 kDa. Altogether these results demonstrate that antiviral sta te induction by IFN-alpha in L1210 cells is preceded by GAbfl binding to the ISRE of the IFN-inducible genes.