We constructed an E. coli strain that produces chloramphenicol acetyltransferase (CAT) in high yield by utilizing the phage T7 RNA polymerase`s stringent recognition and active transcription of T7 promoter. The T7 expression system was constructed such that the T7 RNA polymerase gene is under the control of lacUV5 promoter in one plasmid, and that the target gene, a promoterless CAT gene along with E. coli ribosome binding site is under the control of a T7 promoter in the other plasmid. Without induction the T7 expression system is repressed by lacI repressor. The T7 RNA polymerase produced by IPTG induction is capable of specific and efficient transcription of the CAT gene. Upon IPTG induction CAT is produced up to 36% of the total protein content. The overproduced CAT proteins are soluble but mostly inactive, as assayed for specific activity of the protein.