Regulation in the hut System

Abstract

INTRODUCTION
Although the ability to degrade L-histidine is widely distributed among bacteria, nature has not seen fit to bestow this ability on Escherichia coli. However, enteric organisms related to E. coli, such as Klebsiella aerogenes (Magasanik and Bowser 1955) and some strains of Salmonella typhimurium (Meiss et al. 1969), are capable of degrading histidine to an equimolar mixture of ammonia, L-glutamate, and formamide. A comparison of the hut (histidine utilization) system with lac is therefore reasonable, particularly since it has been possible to introduce the hut genes of K. aerogenes (Tyler and Goldberg 1976) and of S. typhimurium (Smith 1971) into E. coli, where they function essentially as they do in the strains from which they were derived (Parada and Magasanik 1975; Goldberg et al. 1976).

The comparison between hut and lac reveals striking similarities. Both systems are negatively controlled by a specific repressor. In both systems the physiological inducer, L-histidine or β-D-lactose, is converted by an enzyme of the respective system, histidase or β-galactosidase, to the actual inducer, urocanate or allolactose. Both systems contain an inducer-destroying enzyme, urocanase or β-galactosidase. Both systems consist of two linked units of transcription: in each case one of the units contains the structural gene for the repressor. At least one unit of transcription of either system is subject to regulation by catabolite repression.

The differences between hut and lac are equally striking. In hut, the unit of transcription containing the structural gene for the repressor contains, in addition, the structural genes for two...