Growth of Klebsiella aerogenes on xylitol: implications for bacterial enzyme evolution

When Klebsiella aerogenes was grown in continuous culture with xylitol. an unnatural pentitol, as the growth limiting substrate, the structural gene which codes for ribitol dehydrogenase, an enzyme which gratuitously catalyzes the oxidation of xylitol to D-xylulose, was duplicated. It appears that the duplication mechansim only duplicates the gene which is subjected to selective pressure and not any of the other closely linked genes. The degree to which the ribitol dehydrogenase gene is duplicated does not appear to be strictly correlated with the ability to grow faster on xylitol. Duplication mutants do, in fact, grow faster than their parent strain, but when challenged to grow at even higher growth rates there is a catabolic repression of enzyme activity. Thus a situation is created in which a structural gene is duplicated in response to selective pressure; these mutants can grow faster on the new substrate, but faster growth results in a "silencing" of a portion of the genes by catabolite repression.