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The operon model for regulation of enzyme synthesis was proposed by Jacob and Monod in 1961.1, 2 The genetic elements of this control system consist of two parts: a locus, i, which directs the synthesis of a repressor, and a locus called the operon which contains the operator gene and associated structural genes. A messenger RNA molecule can be transcribed from the operon, which is believed to contain in one continuous polynucleotide chain the information for making the polypeptides encoded by the different structural genes. It is not known whether the messenger RNA contains a region corresponding to the operator site and, likewise, it is not known whether the repressor stops protein synthesis by interacting with the DNA, the transcribed mRNA, or a hybrid of both. The inhibiting effect of repressor is reversed by inducer which interacts directly with the repressor. Most available genetic and biochemical data favor the idea that repressors are proteins.2 Confirmation of the repressor-operator model awaits a cell-free biochemical system in which the effect of the repressor on peptide synthesis can be directly demonstrated. In this paper such a system is described for the lac operon of Escherichia coli. Materials and Methods.-E. coli strains: rlF': This strain was derived from strain rl, described by Zipser and Newton,3 by introduction of an episome. rlF' has a deletion of most of the Z gene and carries two i genes, one on the chromosome and one on the episome. This strain was obtained from Dr. D. Zipser. M107: This strain is F-, i-, Z-, a63su+, SmR, and was obtained from Dr. D. Zipser. Z: This strain is HfrH, BI-, lysogenic for 080 and 0801ac, and contains a temperature-sensitive repressor for bacteriophage induction. It was obtained from Dr. W. Gilbert. 21F': This strain was derived from strain M15 described by Ullmann, Jacob, and Monod;4 it has a deletion of about one third of the a portion of the Z gene. Strain 21F' is iand carries that portion of the gene for the Z protein present in strain M15 on both chromosome and episome. It was obtained from Mrs. C. Michels. Preparation of S-30 extract: The cell-free extract of E. coli used in all syntheses is prepared by a modification of the method of Nirenberg5 described elsewhere.6 Preparation of DNA: A partially purified bacteriophage preparation was obtained from strain Z by a modification of the procedure of Kaiser and Hogness7 as described elsewhere.8 The DNA is prepared from the bacteriophage preparation by a minor modification of the procedure of Mandell and Hershey.6 I Incubation conditions for synthesis: All materials which require thawing are warmed to 5°C in a water bath. The incubation mixture contains per ml: 40 jpmoles tris-acetate, pH 8.0; 4.5 Jtmoles 2-mercaptoethanol; 14.6 jmoles magnesium acetate; 7.3 umoles CaCl2; 0.2 jsmole 19 C12amino acids; 2.0 umoles ATP; 0.5 umole each of GTP, CTP, UTP; 18 umoles PEP trisodium salt; 50 u~g PEP kinase; 0.1 umole C14-L-leucine; 200 ug ,k801ac DNA; 50 ,umoles potassium acetate; 100 ug tRNA; and 6500 ug protein as 8-30 extract. All ingredients except the 8-30 extract are mixed together and preincubated for 3 min at 37°C. After adding the S-30 extract, the reaction tubes are incubated at 37°C for 60 min. In experiments where partially purified repressor or IPTG inducer is added, these are mixed with the 8-30 extract and the above procedures are followed.

DNA-directed peptide synthesis. 3. Repression of beta-galactosidase synthesis and inhibition of repressor by inducer in a cell-free system.