Overlapping transcriptional units on the same strand within the murine beta-glucuronidase gene complex

We have identified and partially characterized a complex transcriptional unit within the murine beta-glucuronidase gene complex on chromosome 5. On the same strand and within the first intron of the beta-glucuronidase structural gene, Gus-s, we observe an RNA polymerase II promoter motif. That sequences within this carefully defined region can promote RNA polymerase II transcription is supported by results of in vitro transcriptional runoff assays and by expression of a linked reporter gene in both cultured cells and transgenic mice. Results of RNA blot hybridization and S1 nuclease protection studies reveal a 2.2-kilobase processed liver transcript which is initiated just downstream of the promoter motif and sharing little, if any, sequence with the 2.7-kilobase beta-glucuronidase mRNA. Both RNA species are found in liver where beta-glucuronidase is known to be expressed in all cell types. To our knowledge, this is the first description of eukaryotic mRNAs from overlapping transcription units which share the same strand yet exhibit little, if any, sequence similarity. A possible regulatory relationship between these overlapping structural genes is discussed.