Genomic organization of the human cystathionine beta-synthase gene: evidence for various cDNAs

A CBS cDNA isolated from an adult liver cDNA library was cloned and sequenced. The 5' untranslated region (5'-UTR) contains a sequence which is only partially common (88nt) with that previously published (3). It is expressed as a 2.5kb species mostly in liver and pancreas and faintly in brain, heart, kidney and lung and as a 3.7 kb in pancreas and liver. The human cystathionine beta-synthase gene (CBS) was isolated from a cosmid genomic library and its structure was determined. The CBS gene is at least 23 kb long and is composed of 17 exons. The organization of the human gene is different from that of the rat especially in the 5'-UTR. In the rat gene the ATG is present in exon 1, conversely in the human gene first the ATG is present in exon 3 and second the 5'-UTR contains two different exons 1 (E1a and E1b) linked with exon 2.