Regulation of the β-Glucoside System in Escherchia coli K-12

In Escherichia coli wild-type cells, a mutation at the β-glucoside regulatory gene ( bglR + to bglR − ) leads to simultaneous expression of inducible phospho-β-glucosidase B ( bglB + ) and a β-glucoside-specific species of enzyme II (β-glucoside transport I [ bglC + ]); an additional mutation ( bglS + to bglS4 ) allows these enzymes to be formed constitutively. The bgl alleles have been mapped in the following order: pyrE, bglA, bglB, bglS, bglR, bglC, ilvD . The back mutation in the regulatory allele ( bglR − to bglR + ) caused the cessation of the expression of the bglB + , bglS + or bglS4, bglC + alleles. However, a mutation in a strain with bglB + , bglS4, bglR8, bglC + alleles, at the ini site that lies between the bglS4 and the bglR8 allele, allowed the expression of the bglS4 and bglB + alleles, but showed no affect on the expression of the bglC + allele. It is suggested that the ini mutation possesses a promotor-type function that in the absence of regulatory allele function ( bglR8 ) renews the functioning of only the bglS4 and bglB + alleles. The complementation studies have shown that the bglB + , bglS + or bglS4, bglC + alleles are expressed only in cis to the bglR − allele. In the constitutive strain ( bglB + , bglS4, bglR − , bglC + ), the expressed bglS4 allele formed a soluble product that acts in trans over the bglB + and bglC + alleles and that appears effective only when the bglB + and the bglC + alleles are expressed in cis to the bglR − allele. It thus showed that the constitutive biosynthesis of phospho-β-glucosidase B and β-glucoside transport I is under positive control. Since the regulatory allele bglR − lies between the bglS4 and the blgC + alleles, and acts in cis , it appears that the mutation ( bglR + to bglR − ) allows the initiation of transcription in one direction to express the bglS4, bglB + alleles and in the other to express the bglC + allele. The structural genes bglB and bglC lie adjacent to the regulatory genes bglR and bglS , and the structural genes are coordinately controlled by the regulatory genes. It is, therefore, proposed that the bglB, bglS, bglR, bglC genes form a bgl operon.