Research Article 10: Strains with the Promoter Deletion L1 Synthesize An Altered lac Repressor

Abstract

INTRODUCTION
Several mutations in the lac promoter of E. coli have been isolated (Scaife and Beckwith, 1966; Ippen, Miller, Scaife, and Beckwith, 1968). Three of these, L8, L29, and L37 have been shown to be point mutations of the transition class (Arditti, Scaife, and Beckwith, 1968). A fourth mutation, L1, has been postulated to be a deletion of part of the lac promoter (Miller, Ippen, Scaife, and Beckwith, 1968). L1 does not recombine with either of the three promoter point mutations. In addition, strains with L1 are almost completely constitutive for the lac enzymes. In the presence of IPTG there is only a slight increase in the amount of β-galactosidase, while wild type is induced nearly 1000-fold. This is consistent with L1 being a deletion extending from the i gene into the lac promoter.

In this paper we show that the constitutivity of the lac operon in strains with L1 is indeed due to a lesion on the i gene, the structural gene for the lac repressor. We report genetic evidence which demonstrates the existence of an altered lac repressor in strains with L1. This molecule can be detected in vitro and partially purified. We offer an experiment which shows that the L1-repressor is physically different from the wild-type repressor.

MATERIALS AND METHODS
Strains: The construction of F'lacpro episomes with i^Q, and the double mutation i^Q,L1 has been described (Miller, Müller-Hill, and Beckwith, 1968). 115, and SQ are mutations which allow an overproduction of the lac repressor by 35- and...