Replacement of Thymidine by a Modified Base in the Escherichia coli Genome.

Prokaryotic and eukaryotic genomic DNA is comprised of the four building blocks A, G, C, and T. We have begun to explore the consequences of replacing a large fraction or all of a nucleoside in genomic DNA with a modified nucleoside. As a first step we have investigated the possibility of replacement of T by 2'-deoxy-5-(hydroxymethyl)uridine (5hmU) in the genomic DNA of Escherichia coli. Metabolic engineering with phage genes followed by random mutagenesis enabled us to achieve approximately 75% replacement of T by 5hmU in the E. coli genome and in plasmids.