Structural organization of the gene encoding rat cystatin beta.

A genomic DNA clone encompassing the gene (cy beta) encoding rat cystatin beta (Cy beta) was isolated by screening with a rat cy beta cDNA as a probe. The gene spans about 2.6 kb and comprises three exons. The first intron is located between Lys22 and Val23 and the second between Lys56 and Val57 in the deduced amino acid sequence of Cy beta. The second exon contains the highly conserved QVVAG sequence which, unlike the sequence of other cystatin family members, is not split by an intron. In the 5'-upstream region, three SP-1-binding sites exist, but no typical TATA-box or CAAT-box sequences are found. The difference in the organization of the rat cy beta gene, encoding a family-1 cystatin, from that encoding members of the other cystatin families, suggests that cy beta diverged from a common ancestral gene earlier than the separation of genes encoding family-2 and family-3 cystatins.