<P> The researchers also found that all Cit clones had mutations in which 2933 base pair segment of DNA was duplicated or amplified . The duplicated segment contained the gene for the citrate transporter protein used in anaerobic growth on citrate, citT . The duplication is tandem, and resulted in copies that were head - to - tail with respect to each other . This new configuration placed a copy of the previously silent, unexpressed citT under the control of the adjacent rnk gene's promoter, which directs expression when oxygen is present . This new rnk - citT module produced a novel regulatory pattern for citT, activating expression of the citrate transporter when oxygen was present, and thereby enabled aerobic growth on citrate . </P> <P> Movement of this rnk - citT module into the genome of a potentiated Cit clone was shown to be sufficient to produce a Cit phenotype . However, the initial Cit phenotype conferred by the duplication was very weak, and only granted a ~ 1% fitness benefit . The researchers found that the number of copies of the rnk - citT module had to be increased to strengthen the Cit trait sufficiently to permit the bacteria to grow well on the citrate . Further mutations after the Cit bacteria became dominant in the population continued to accumulate improved growth on citrate . </P> <P> The researchers concluded that the evolution of the Cit trait occurred in three distinct phases in which mutations accumulated that increased the rate of mutation to Cit, the trait itself appeared in a weak form, and, finally, the trait was improved by later mutations . Blount et al. suggested that this pattern might be typical of how novel traits in general evolve, and proposed a three - step model of evolutionary innovation: </P> <Ol> <Li> Potentiation: a genetic background evolves in which a trait is mutationally accessible, making the trait's evolution possible . </Li> <Li> Actualization: a mutation occurs that produces the trait, making it manifest, albeit likely in a weak form . </Li> <Li> Refinement: Once the trait exists, if it provides selective benefit, mutations will accumulate that improve the trait, making it effective . This phase is open - ended, and will continue so long as refining mutations arise and the trait remains beneficial . </Li> </Ol>

Researchers found e coli that had mutation rates