Cystathionine beta -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 -Ib 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)l 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 Drosophiln SL2 cells, a functional interplay was indicated between NF-Y binding to the CAAT-box, or between USF-I 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 Spl 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 Downs syndrome.