A 41-nucleotide-long duplex DNA, which contains the translation termination codon TAA in six reading frames and lactose operator sequence of Escherichia coli, has been synthesized. This fragment may be useful not only for producing a truncated protein encoded in a plasmid, but also for the identification of the precise coding region and translation direction of a bacterial gene in the cloned chromosomal segment. The synthetic fragment was inserted into beta-lactamase structural gene in pBR322 in order to test the in vivo activity. The plasmid produced mutant beta-lactamase reduced in size, as expected from the insertion site, and rendered the host bacterium constitutive for beta-galactosidase. Thus, termination codons and lactose operator in synthetic nucleotide appear to be functional in vivo.