Efficient expression of the Escherichia coli leuB (beta-isopropylmalate dehydrogenase) gene occurred in yeast after in vitro DNase digestion and religation of plasmid bound leuB and the yeast HIS3 DNA which placed the 5' end of the yeast HIS3 gene immediately adjacent to the coding region of the E. coli leuB gene. Two structurally distinct classes of gene fusions were constructed, each involved portions of the yeast HIS3 gene which contributed DNA sequences responsible for leuB expression in yeast. The first class involved fusion of the HIS3 coding region to bacterial DNA resulting in the production of a fusion protein with beta-isopropylmalate dehydrogenase activity. The second class consisted of bacterial DNA, including the leuB coding region, fused to the HIS3 promotor region with the absence of any portion of the HIS3 coding region. In both constructions the HIS3 promotor region is required for transcription, however, translation of the class two fusion is initiated at a bacterial DNA coded AUG, and the 5' end of the mRNA coded by the leuB gene mapped predominantly at bacterial DNA sequences. The DNA sequence responsible for the 5' end of the HIS3 mRNAs remain in the class two gene fusions but this did not preclude the initiation of transcription at bacterial DNA sequences. The pattern of mRNA initiation at bacterial DNA suggests that DNA sequences at, or adjacent to, the site of transcription initiation are involved in the determination of the sites of initiation, and perhaps the frequency at which initiation occurs.