Tissue-specific expression and differential inducibility of several microsomal epoxide hydrolase mRNAs which are formed by alternative splicing.

mRNA was isolated from several rat tissues and subjected to either the nuclease S1 or the RNAseA protection assay with probes covering the 5' end, the middle part, and the 3' end of the microsomal epoxide hydrolase (mEHb) cDNA. Whereas probes directed against the latter two regions yielded a single protected fragment, a probe which covered base pairs -148 to +453 (+1 defines the start of protein biosynthesis) yielded two protected fragments. The degree of protection of the two fragments was strongly dependent on the tissue from which the mRNA had been isolated. Thus at least two mEHb mRNAs which differ at their 5' ends are differentially expressed in various tissues. In addition the mRNAs corresponding to the two protected fragments were clearly differentially inducible by Aroclor 1254 treatment of the animals. Primer extension analysis with hepatic RNA from untreated animals yielded three primer-extended products corresponding to three mRNAs which differ at their 5' ends. As already seen in the nuclease S1 protection assay, one of the mRNAs was induced by Aroclor 1254 treatment. The expression of the two other mRNAs was either repressed or stable. Thus besides the mRNA already characterized for mEHb, there are at least two other mEHb mRNAs. This result was confirmed by the isolation of a mEHb cDNA which is completely distinct in its sequence in a region just preceding the initiation codon for protein biosynthesis. From that point on, the sequence of our cDNA becomes identical to the published mEHb cDNA. This point corresponds exactly to the start of exon 2 as determined from the genomic sequence. Thus the region where both mEHb cDNAs differ is encoded by two different exons 1, which are joined to exon 2 by alternative splicing. The tissue-specific expression and the different inducibility of the various mEHb mRNAs might indicate that their expression is governed by different promoters.