Yeast HIS3 expression in Escherichia coli depends upon fortuitous homology between eukaryotic and prokaryotic promoter elements.

The yeast imidazoleglycerolphosphate dehydratase gene HIS3, when introduced into Escherichia coli, is transcribed and translated with sufficient fidelity to produce functional enzyme. The following lines of evidence indicate that E. coli RNA polymerase recognizes a particular region of HIS3 DNA as a promoter sequence. First, this promoter contains nucleotide sequences that resemble the canonical prokaryotic promoter elements, the -10 and -35 regions. Second, HIS3 transcription in vitro by E. coli RNA polymerase is initiated at the predicted site downstream from the conserved sequences. Third, deletion mutations that successively encroach upon the 5' end of the HIS3 gene indicate that the promoter is necessary and sufficient for expression in E. coli. Fourth, a single base-pair change that behaves as an "up-promoter" mutation alters the -35 region such that it becomes identical with the consensus sequence. Because the -10 region of this promoter coincides with the TATA promoter element that is necessary for expression in yeast cells, it is possible directly to compare prokaryotic and eukaryotic promoter function. Analysis of 51 deletion and substitution mutations indicates that the patterns of mutant phenotypes are quite different for each organism. Therefore, although prokaryotic -10 regions are similar in sequence to eukaryotic TATA elements and although the same his3 region serves both functions, it appears that this represents an evolutionary coincidence whose current functional basis is minimal. The evolutionary significance of the homology between prokaryotic and eukaryotic promoter elements is discussed.