Directed evolution of cellobiose utilization in Escherichia coli K12.

The cellobiose catabolic system of Escherichia coli K12 is being used to study the role of cryptic genes in evolution of new functions. Escherichia coli does not use beta-glucoside sugars; however, mutations in several loci can activate the cryptic bgl operon and permit growth on the beta-glucoside sugars arbutin and salicin. Such Bgl+ mutants do not use cellobiose, which is the most common beta-glucoside in nature. We have isolated a Cel+ (cellobiose-utilizing) mutant from a Bgl+ mutant of E. coli K12. The Cel+ mutant grows well on cellobiose, arbutin, and salicin. Genes for utilization of these beta-glucosides are located at 37.8 min on the E. coli map. The genes of the bgl operon are not involved in cellobiose utilization. Introduction of a deletion covering bgl does not affect the ability to utilize cellobiose, arbutin, or salicin, indicating that the new Cel+ genes provide all three functions. Spontaneous cellobiose negative mutants also become arbutin and salicin negative. Analysis of beta-glucoside positive revertants of these mutants indicates that there are separate loci for utilization of each of the beta-glucoside sugars. The genes are closely linked and may be activated from a single locus. A fourth gene at an unknown location increases the growth rate on cellobiose. The cel genes constitute a second cryptic system for beta-glucoside utilization in E. coli K12.