Environmental Control of Microbial Gene Expression and Evolution

Summary

The survival of microorganisms is dependent on their ability to respond to a changing environment. In the very stressed environment of the CF lung, with salty and dehydrated mucus, the microorganisms need to protect themselves from losing their intracellular water and one way to accomplishing this is to produce an exopolysaccharide capsule with strong gelling properties as a barrier to dehydration. It is interesting that the algD promoter is activated by those environmental factors that are characteristic of the CF disease, which explains why CF patients are particularly vulnerable to infections by mucoid P. aeruginosa. It is also interesting to note that the alginate capsule, which is presumably produced to protect P. aeruginosa from intracellular dehydration, also affords protection The activation of the algD promoter has been measured by measuring catechol 2,3-dioxygenase (C230) activity, since a construct containing the xylE gene placed under the algD promoter was used in these experiments (see ref. 18 and 19). One unit of catechol 2,3-dioxygenase activity is defined as the amount of enzyme oxidizing one mmol of catechol to 2-hydroxymuconic semialdehyde, a product with a molar extinction coefficient of 4.4 × 10⁴ at 375 nm. against antibiotics and antibodies to the detriment of the human patients. When the environment is beset with chlorinated compounds, the immediate response of natural microorganisms is to evolve degradative genes in the form of a plasmid. As a first step, they tend to recruit genes that allow degradation of a structurally analogous non-chlorinated compound, which undergo mutational or recombinational divergence to the appropriate genes whose products have broad substrate specificities to include chlorinated compounds. Such evolutionary processes apparently are not very effective for highly chlorinated compounds for which appropriate enzyme systems have not yet fully developed. In a single incidence of directed evolution, the chromosomal DNA shows the presence of multiple copies of a transposable element near the evolved genes, suggesting that an accelerated process of evolution may bypass the requirement of genetic relatedness of the evolved genes to the genome of the recruiting cells.