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Chapter II: Lac: The Genetic System

Jonathan R. Beckwith


This chapter will deal with the methods and tools used for genetic analysis of the lac* operon. The specific problems which arise in the genetic analysis of this system are, in many cases, similar to those encountered in other genetic regulatory units. Therefore, the solutions to these problems can provide a methodology applicable to at least certain of these other systems. The generality of the techniques and approach will be evaluated in this article. A more direct application of these techniques derives from the existence of strains in which the lac genes are fused to other bacterial operons, such as the trp operon and purE operon (Beckwith, Signer, and Epstein, 1966; Jacob, Ullmann, and Monod, 1965). Since, in such strains, the lac genes are now part of the other operon, nearly all the methodology used for analysis of lac can be used for that operon. For example, from the lac-trp fusion strains, mutations in the trpR regulatory gene can be easily selected using techniques normally employed in lac genetics (Reznikoff, unpublished results).

Most of the major concepts of operon structure, expression and regulation have come out of the work on the lac operon of E. coli. That lac has generated so many of these ideas is due to several factors: (1) The happy coincidence of Monod and Jacob working in the same institute, thus affording a collaboration between two creative minds in the fields of bacterial physiology and regulation and bacterial genetics. The interplay of these fields has...

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