Transsulfuration in Saccharomyces cerevisiae is not dependent on heme: purification and characterization of recombinant yeast cystathionine beta-synthase

Cystathionine beta-synthase [CBS; L-serine hydro-lyase (adding homocysteine), EC 4.2.1.22] catalyzes the first committed step of transsulfuration in both yeast and humans. It has been established previously that human CBS is a hemeprotein but although the heme group appears to be essential for CBS activity, the exact function of the heme group is unknown. CBS activity is absent in heme deficient strains of Saccharomyces cerevisiae grown without heme supplementation. CBS activity can be restored by supplementing these strains with heme, implying that there is a heme requirement for yeast CBS. We subcloned, overexpressed and purified yeast CBS. The yeast enzyme shows absolute pyridoxal 5'-phosphate (PLP) dependence for activity but we could find no evidence for the presence of a heme group. Given the degree of sequence and mechanistic similarity between yeast and human CBS, this result indicates that heme is unlikely to play a direct catalytic role in the human CBS reaction mechanism. Further characterization revealed that, in contrast to human CBS, S-adenosylmethionine (AdoMet) does not activate yeast CBS. Yeast CBS was found to be coordinately regulated with proliferation in S. cerevisiae. This finding is the most likely explanation of the observed apparent heme dependence of transsulfuration in vivo.

The human cystathionine beta-synthase (CBS) gene: complete sequence, alternative splicing, and polymorphisms

Cystathionine beta-synthase [CBS; l-serine hydro-lyase (adding homocysteine), EC 4.2.1.22] catalyzes the first committed step of transsulfuration and is the enzyme deficient in classical homocystinuria. In this report, we describe the molecular cloning and the complete nucleotide sequence of the human CBS gene. We report a total of 28,046 nucleotides of sequence, which, in addition to the CBS gene, contains approximately 5 kb of the 5' flanking region. The human CBS gene contains 23 exons ranging from 42 to 209 bp. The 5' UTR is formed by 1 of 5 alternatively used exons and 1 invariably present exon, while the 3' UTR is encoded by exons 16 and 17. We also describe the identification of two alternatively used promoter regions that are GC rich (approximately 80%) and contain numerous putative binding sites for Sp1, Ap1, Ap2, and c-myb, but lack the classical TATA box. The CBS locus contains an unusually high number of Alu repeats, which may predispose this gene to deleterious rearrangements. Additionally, we report on a number of DNA sequence repeats that are polymorphic in North American and European Caucasians.

Differential effects of sodium butyrate on the transcription of the human TIS11 family of early-response genes in colorectal cancer cells

The tetradecanoyl phorbol acetate (TPA)-inducible sequence 11 (TIS11) family of early-response proteins consists of at least five members. They share a highly conserved Cys3His zinc-binding motif, but otherwise have little sequence similarity. Their function remains unknown, but all are induced rapidly and transiently in response to extracellular hormone and growth factor signals. Sodium butyrate, a fermentation product of dietary fibre, effects colorectal cancer cell proliferation by inducing growth arrest, differentiation and apoptosis. In this communication, we report that butyrate has differential effects on the transcription of the three human TIS11 family members identified so far in T84 and HT-29 human colorectal cancer cell lines. Butyrate response factor 1 (BRF1) transcription is repressed, butyrate response factor 2 (BRF2) transcription is activated and there is no apparent effect on the transcription of human TIS11 (HTIS11). Induction and repression occur rapidly, with altered mRNA levels detectable within 15 min of butyrate addition. Two other short-chain fatty acids, propionate and acetate, have no detectable effects on BRF1 or BRF2 transcription.

The human immediate early gene BRF1 maps to chromosome 14q22-q24

BRF1 (Butyrate response factor 1) is a member of an immediate early gene family specifying putative nuclear transcription factors. A repeat motif incorporating two Cys and two His is highly conserved between family members identified from yeast, Drosophila, mouse, rat, and human. The chromosome localization of none of the human genes has been determined thus far. Using the polymerase chain reaction on a human-rodent hybrid panel, we have localized BRF1 to chromosome 14. This was confirmed by direct sequencing of the PCR fragment. Using fluorescence in situ hybridization, the chromosome localization of BRF1 was further determined as 14q22-q24.

ERF-2, the human homologue of the murine Tis11d early response gene

A human cDNA specifying a member of the Tis11 early response gene family was cloned and sequenced. The human gene differs from its mouse homologue by encoding an additional 97 amino acids at its C-terminal end. The sequence has transactivation-like motifs, an unusual Cys-Ser-Ala-rich motif and displays sequence similarity at the extreme C-terminal end with another Tis11 family member, ERF-1.