Evolution of a new enzymatic function by recombination within a gene.

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Abstract
Mutations that alter the ebgA gene so that the evolved .beta.-galactosidase (ebg) enzyme of Escherichia coli can hydrolyze lactose fall into 2 classes: class I mutants use only lactose, whereas class II mutants use lactulose and lactose. Neither class uses galactosylarabinose effectively. When both a class I and a class II mutation are present in the same ebgA gene, ebg enzyme acquires a specificity for galactosylarabinose. Although galactosylarabinose utilization can evolve as the consequence of sequential spontaneous mutations, it can also evolve via intragenic recombination in crosses between class I and class II ebgA+ mutant strains. The sites for class I and class II mutations lie about 1 kilobase, or about 1/3 of the gene, apart in ebgA. Implications of these findings with respect to the evolution of new metabolic functions are discussed.