Z-DNA in the rat somatostatin gene

Two alternating purine-pyrimidine tracts flank the rat gene encoding the polypeptide hormone somatostatin. One lies 5' and one 3' to the gene; both consist of tandem repeats of the dinucleotide TG, with 25 repeats in the tract 5' and 15 in the tract 3' to the gene, and both are bordered on at least one edge by short repeated sequences. Characterization of supercoiled plasmids containing these sequences reveals that both form Z-DNA. Using S1 nuclease as a probe of DNA conformation we have investigated the fine structure of the Z-DNA and have shown: 1) that the entire Z-DNA segment as well as the single-stranded junctions flanking it is sensitive to S1 nuclease; 2) that the B-DNA/Z-DNA junction can be contained within the ends of the alternating purine-pyrimidine tract; and 3) that the sequences bordering the alternating purine-pyrimidine tracts affect the extent of Z-DNA propagation, sometimes as a result of their own apparently nonB-DNA conformation. We have also examined the published sequence of the human somatostatin gene (Shen, L.-P., and Rutter, W.J. (1984) Science 224, 168-171) for alternating purine-pyrimidine or potential Z-DNA-forming sequences and compared them to those present in the rat gene. We find that the human gene contains a 32-base pair alternating purine-pyrimidine sequence in an analogous position to the (TG)25 tract 5' to the rat gene, although the two sequences are not homologous. There are also six shorter alternating purine-pyrimidine elements 5' to the transcribed sequences which are positioned almost identically in the two genes with respect to the transcription initiation sites, although their sequences are not well conserved. We propose that the parallel placement of alternating purine-pyrimidine or potential Z-DNA-forming sequences 5' to the somatostatin genes from two species is a result of structural, in contrast to sequence, conservation. These observations suggest that the rat somatostatin gene may be a good model system for the investigation of the function of Z-DNA in the regulation of gene expression.