Transcriptional regulation of the human cystathionine β-synthase - 1b basal promoter: Synergistic transactivation by transcription factors NF-Y and Sp1/Sp3

Abstract

Cystathionine β-synthase (CBS) catalyses the condensation of serine and homocysteine to form cystathionine, an intermediate step in the synthesis of cysteine. Human CBS encodes five distinct 5′ non-coding exons, the most frequent termed CBS - 1a and CBS - 1b, each transcribed from its own unique GC-rich TATA-less promoter. The minimal transcriptional region (- 3792 to - 3667) of the CBS - 1b promoter was defined by 5′and 3′-deletions, and transient transfections of reporter gene constructs in HepG2 cells, characterized by CBS transcription exclusively from the - 1b promoter. Included in this 125 bp region are 3 GC-boxes (termed GC-a, GC-b and GC-c), an inverted CAAT-box and an E-box. By gel-shift and supershift assays, binding of specificity protein (Sp)1 and Sp3 to the GC-box elements, upstream stimulatory factor 1 (USF-1) to the E-box, and both nuclear factor (NF)-Y and an NF-1-like factor to the CAAT-box could-be demonstrated. By transient transfections and reporter gene assays in HepG2 and Drosophila SL2 cells, a functional interplay was indicated between NF-Y binding to the CAAT-box, or between USF-1 binding to the E-box, and Sp1/Sp3 binding to the GC-box elements. In SL2 cells, NF-Y and Sp1/Sp3 were synergistic. Furthermore, both Sp1 and the long Sp3 isoform transactivated the CBS - 1b minimal promoter; however, the short Sp3 isoforms were potent repressors. These results may explain the cell- or tissue-specific regulation of CBS transcription, and clarify the bases for alterations in CBS gene expression in human disease and Down's syndrome.