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Binding of the ubiquitous cellular transcription factors Sp1 and Sp3 to the ZI domains in the Epstein-Barr virus lytic switch BZLF1 gene promoter.

Induction of the Epstein-Barr virus lytic cycle in latently infected B cells requires the expression of the immediate-early lytic gene BZLF1. We have previously identified several cis-elements within the BZLF1 promoter that are required for induction by known inducers of the lytic cycle [E. Flemington and S. H. Speck (1990)J. Virol. 64, 1217-1226]. These include four elements termed the ZI domains (ZIA, ZIB, ZIC, and ZID) that share extensive homology and that have recently been shown to bind several cellular transcription factors [A. M. Borras, J. L. Strominger, and S. H. Speck (1996) J. Virol. 70, 3894-3901]. Here Sp1 and Sp3 are identified as the cellular factors present in crude B cell nuclear extract preparations that bind to the ZIC domain. In addition, three of the four complexes observed in electrophoretic mobility shift analyses employing probes containing either the ZIA or the ZID domains also represent Sp1 or Sp3 binding. Binding of Sp1 and Sp3 to the ZI domains was shown to be significantly weaker than binding of these factors to a consensus Sp1 site. A heterologous promoter construct containing three repeats of a consensus Sp1 site, cloned upstream of a single copy of the ZII (CREB/ AP1) element from the BZLF1 promoter linked to the beta-globin TATA box, exhibited phorbol ester inducibility. The latter observation was consistent with the functional behavior exhibited by a heterologous promoter construct containing multiple copies of the ZIC domain liked to the ZII element. However, the basal activity of the heterologous promoter construct driven by the consensus Sp1 sites was ca. 10-fold higher than that of the heterologous reporter construct containing multimerized ZIC sites. Thus, the low affinity of Sp1 binding to the ZI domains may contribute to the low-level basal activity of the BZLF1 promoter.