Sequence requirements for transcriptional arrest in exon 1 of the human adenosine deaminase gene.

We have previously demonstrated that a transcriptional arrest site exists in exon 1 of the human adenosine deaminase (ADA) gene and that this site may play a role in ADA gene expression (Z. Chen, M. L. Harless, D. A. Wright, and R. E. Kellems, Mol. Cell. Biol. 10:4555-4564, 1990). Sequences involved in this process are not known precisely. To further define the template requirements for transcriptional arrest within exon 1 of the human ADA gene, various ADA templates were constructed and their abilities to confer transcriptional arrest were determined following injection into Xenopus oocytes. The exon 1 transcriptional arrest signal functioned downstream of several RNA polymerase II promoters and an RNA polymerase III promoter, implying that the transcriptional arrest site in exon 1 of the ADA gene is promoter independent. We identified a 43-bp DNA fragment which functions as a transcriptional arrest signal. Additional studies showed that the transcriptional arrest site functioned only in the naturally occurring orientation. Therefore, we have identified a 43-bp DNA fragment which functions as a transcriptional arrest signal in an orientation-dependent and promoter-independent manner. On the basis of our findings, we hypothesize that tissue-specific expression of the ADA gene is governed by factors that function as antiterminators to promote transcriptional readthrough of the exon 1 transcriptional arrest site.