We have examined the transcription in yeast of Escherichia coli-yeast 2-micrometers DNA recombinant plasmids carrying the bacterial beta-lactamase (bla) gene. In Saccharomyces cerevisiae both strands of the gene are transcribed giving multiple RNA species of distinct lengths. At least one RNA transcript derived from the coding strand initiates at a yeast promoter on the 2-micrometers DNA segment. Another mRNA of 1.1 kb starts right in front of the gene on the bacterial DNA sequence. Deletion experiments have shown that expression of the bacterial bla gene is dependent on the presence of bacterial sequences right in front of the gene. Mutants lacking the bacterial promoter region do not give detectable gene products in yeast. The expression can be restored by substituting for the deleted sequence a DNA fragment which carries the E. coli lac promoter-operator region. We conclude that the bacterial promoter region of the bla gene as well as the lac promoter-operator fragment have promoter activity in yeast and that yeast-bla fusion transcripts cannot be used as a functional messenger for beta-lactamase in yeast.