The rat liver genome contains sequences homologous to sequences within pUC and M13 DNA

Tyrosine aminotransferase (TAT) is a liver specific enzyme that is subject to complex regulation by cAMP, glucocorticoids, and insulin. Induction of the enzyme by cAMP and glucocorticoids has been shown to be due to increased levels of TAT mRNA resulting from an enhanced rate of transcription. However, reports on the mechanism of induction by insulin have been conflicting. Possible mechanisms of insulin action have been shown at the level of transcription, post-transcription, and post-translation. It may be that the mechanism of insulin depends upon the cell type. With respect to insulin action at the level of transcription, it has been shown in rat hepatocytes and in FT0-2B hepatomas that insulin has no affect on transcription. However in adult rat liver it was shown that insulin caused a direct increase in the rate of transcription of TAT. In H-35 cells it has been reported that insulin causes a transient decrease in the rate of transcription. It has been shown in a variety of genes that specific genomic sequences are necessary for the regulation of transcription by both cAMP and glucocorticoids. It has also been suggested, but not reported, that other sequences are necessary for the regulation of transcription by insulin. Thus, it was the purpose of this study to isolate and characterize a genomic clone to TAT. Using various recombinant DNA techniques and transfection assays, sequences of the gene could then be tested for possible insulin regulatory elements. However, in the processs of characterizing two clones it was discovered that sequences located within the the vector DNA of the probes detected homology in genomic DNA of the rat liver library. A possible explanation for this surprising observation is discussed.