Regulation of histone gene transcription at the G1/S phase transition via the Site II cell cycle control element is distinct from E2F-dependent mechanisms operative at the growth factor-related restriction point. E2F-independent activation of histone H4 gene expression combines contributions of several promoter factors, including HiNF-M/IRF2 and the HiNF-D/CDP-cut complex which contains pRB, CDK1, and cyclin A as non-DNA binding subunits. Mutational analyses suggest additional rate-limiting factors for Site II function. Using sequence-specific Site II DNA affinity chromatography, we identified a 45 kDa protein (KIAA0005 or BZAP45) that is embryonically expressed and phylogenetically conserved. Based on amino acid sequence analysis, BZAP45 contains a unique decapeptide that is part of a putative leucine-zipper protein with a nucleotide (ATP or GTP) binding fold. Bacterial expression of a full-length cDNA produces a 45 kDa protein. Binding studies reveal that highly purified BZAP45 does not interact with Site II, suggesting that BZAP45 function may require partner proteins. Forced expression of BZAP45 strongly stimulates H4 promoter (nt -215 to -1)/CAT reporter gene activity. Deletion analyses and point mutations indicate that BZAP45 enhances H4 gene transcription through Site II. Thus, BZAP45 is a novel regulatory factor that contributes to transcriptional control at the G1/S phase transition.
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