Purification and characterization of the nuclear factor BA1. A transcriptional activator of the human apoB gene.

The promoter region of human apoB -79 to -63 is recognized by a sequence-specific DNA binding protein, designated NF-BA1, which is essential for the transcriptional activation of the apoB gene in hepatic and intestinal cells. This protein has been purified to apparent homogeneity from rat liver nuclear extracts. The purification steps involve Q-Sepharose, Bio-Rex 70, S-Sepharose, and DNA-specific affinity chromatography. The purified protein was identified as a polypeptide of 60 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and photoaffinity cross-linking. In vitro transcription complementation experiments using normal and mutated -268 to +8 apoB promoter sequences as templates indicated that the purified protein retained the ability to stimulate apoB transcription when added to hepatic nuclear extracts. The stimulation of transcription is associated with the binding of the factor to its recognition sequence. A synthetic promoter consisting of five contiguous -79 to -63 elements in front of the core apoB promoter region -38 to +8 is strongly activated by NF-BA1 protein in hepatic extracts in vitro. Footprinting analysis showed that purified NF-BA1 binds to the regulatory regions of apoCIII (-87 to -63), apoAII (-740 to -719), and apoAI (-212 to -191) genes and may be involved in the regulation of their expression.