New Aspects of Thymidine Metabolism in E. coli

Emory University, Atlanta GA

Investigators

Abstract

With a single known exception (a virus), the DNA, or genetic material, of every organism contains thymine rather than its simpler analog, uracil. This is true even though uracil was used long before thymine in evolution and even though, in the test tube, uracil-containing DNA can be replicated and encodes genetic traits. The reason for the preference of thymine over uracil is a major unanswered question in biology. A major long term goal of this project is to produce a thymineless bacterial chromosome. A multiple mutant in a common gastrointestinal bacterium (Escherichia coli) has been already been constructed that can replace 30% of its chromosomal thymine with uracil. By gradually forcing this bacterium to use increasing amounts of an alternative DNA precursor, deoxyuridine, from the medium, it may be possible to drive the evolution of a thymineless chromosome. The bacterium used in this project has been widely used for the discovery of universal metabolic pathways. It has multiple ways to make thymine compounds, to keep uracil out of DNA, and to remove it once it is incorporated. Some pathways that are still unknown will be identified by mutation. By successive mutation, it may be possible to "reverse" evolution and produce a bacterium that no longer excludes uracil from the chromosome. Whether or not the goal of a completely thymineless chromosome is reached, the results should provide valuable information about alternate pathways of thymine synthesis, about what genes and enzymes keep uracil out of DNA, and about differences in the biological properties of uracil-containing versus thymine-containing DNA.