In vitro binding of cell-specific and ubiquitous nuclear proteins to the octamer motif of the SV40 enhancer and related motifs present in other promoters and enhancers.

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Abstract
We have used the gel retardation and DNase I footprinting assays to investigate the in vitro binding of nuclear proteins to the octamer motif present in domain A of the SV40 enhancer and in other enhancer and promoter elements. Three apparently cell‐specific (oct‐B1A, oct‐B1B and oct‐B2) and one ubiquitous (oct‐B3) proteins were detected in various lymphoid and non‐lymphoid cell extracts. We show that the previously described ‘ubiquitous’ NF‐A1 factor may correspond in fact to two proteins, oct‐B1A in HeLa cells and oct‐B1B in lymphoid cells. Interestingly, the HeLa cell protein oct‐B1A formed a complex with the SV40 octamer, which could be detected in gel retardation, but not in DNase I footprinting assays. This absence of protection from DNase I digestion correlates with the inactivity of the SV40 octamer in HeLa cells in vivo. We have also found that the in vitro interaction between the SV40 octamer motif and the lymphoid cell‐specific protein oct‐B2 was negatively modulated by a component present in the nuclear extracts from several lymphoid cell lines. The interactions between the multiple octamer‐binding proteins and the related octamer motifs present in other promoter and enhancer elements were systematically compared and the possible role of these proteins in the control of transcription is discussed.