Untersuchungen zur Induktion der Lac‐Enzyme: 1. Induktionswirkung und Permeation

Abstract

The kinetics of β‐galactosidase induction were investigated in two strains of Escherichia coli, K12 3000 (i+z+y+) and K12 20010 (i+z+y−). The use of a new series of β‐galactoside analogues has led to a better understanding of the induction process. The unique properties of galactosyl glycerol as an inducer have been explained by the existence of a specific permease for this galactoside. Previous studies of the relative efficiency of isopropyl thiogalactoside and thiogalactosyl glycerol as inducers were carried out in y+ strains. The possibility of these inducers having differing affinities for the lac permease, made the earlier observations ambiguous. These experiments have therefore been repeated using a y− strain. Isopropyl thiogalactoside was found to be a better inducer than thiogalactosyl glycerol. An evaluation of the importance of various chemical groupings as determinants of the relative efficacy of inducers has been begun. The 6‐hydroxymethyl group of the galactosyl moiety of β‐galactosides appears to be of considerable importance. The substitution by a methyl group (thiofucosyl glycerol) or by a hydrogen atom (thioarabinosyl glycerol) of the 6‐hydroxymethyl group (thiogalactosyl glycerol) results in the conversion of a compound which is an excellent inducer into a compound which cannot function as inducer at all. A comparison of S‐ and O‐ga‐lactosides shows that, when an O‐galactoside is an inducer, the corresponding S‐galactoside is normally a better inducer. Thioallolactose is an exception to this rule. Despite the fact that it is a weak inducer of β‐galactosidase, thioallolactose is an excellent inhibitor of the lac permease. When the non‐inducer, thioallolactose, was reduced to thioallolactitol, it became a normal “inducteur gratuit”. Since it has been observed that galactose itself can serve as an inducer of β‐galactosidase in mutants lacking galactokinase, the inducer properties of the analogue 1,5‐anhydro‐galactitol were investigated. This compound, although not a good inducer, was superior to galactose. Galactosyl galactoside exhibits similar kinetics of induction to those of lactose. It is suggested that this compound, like lactose, is not itself an inducer but that it is converted to the actual inducer by the basic level β‐galactosidase, which is found in uninduced cells. Galactosyl glycerol has previously been characterised as a uniquely active inducer in that (a) it can act at concentrations as low as 10−6 M. (b) The induction process is without an autocatalytic phase. Observations made in the present study indicate that these properties result from the presence of a seemingly constitutive permease, which is specific for galactosyl glycerol. This permease then favours the accumulation of galactosyl glycerol under conditions of low inducer concentration, when the general lac permease is not present. When the lac permease has been induced, a variety of inducers (including galactosyl glycerol) can be concentrated. The specificity of the galactosyl glycerol permease is emphasized by its lack of transport of thiogalactosyl glycerol or secondary galactosyl glycerol. Copyright © 1967, Wiley Blackwell. All rights reserved