DNA rearrangements that activated a promoterless his3 gene were selected in vivo. DNA segments that promote the expression of his3 were identified in Ty1 DNA sequences and a variety of sites in the vector DNA. These elements appear to function when placed in either orientation relative to his3 but not when placed at the 3' end of the his3 gene. Promoter elements regulated by carbon source, nitrogen source, pyrimidines or galactose were characterized. The assembly of more complex regulatory elements and transposons from these units is discussed.