Expression of a cloned Saccharomyces cerevisiae gene (URA1) is controlled by a bacterial promoter in E. coli and by a yeast promoter in S. cerevisiae.

The expression of a cloned yeast URA1 gene in Escherichia coli and in Saccharomyces cerevisiae was studied. In E. coli, only one orientation of the cloned yeast DNA segment inserted into the bacterial vector (pBR322) allows URA1 expression. Moreover, the permissive orientation changes with the cloning site. The absence of URA1 expression in E. coli can be corrected by the spontaneous integration into the cloned yeast DNA of a 0.9-kb bacterial DNA. Several copies of such a bacterial IS element have been detected in the host E. coli genome. The results strongly suggest that, in E. coli, transcription of the yeast URA1 needs a prokaryotic promoter for its initiation. In S. cerevisiae, the expression of non-chromosomally cloned URA1 does not depend on the orientation of the cloned fragment. Furthermore, it remains under the control of a nuclear regulatory gene (pprX-1) which constitutively enhances the expression of URA1 as well as URA3 at the transcriptional level. Therefore, in S. cerevisiae, transcription of non-chromosomally cloned URA1 involves a physiological yeast promoter cloned along with the structural part of the gene.