Only a Small Subset of the Horizontally Transferred Chromosomal Genes in Escherichia coli Are Translated into Proteins*□S

Horizontally transferred genes are believed to play a crit-ical role in the divergence of bacterial strains from a common ancestor, but whether all of these genes express functional proteins in the cell remains unknown. Here, we used an integrated LC-based protein identification tech-nology to analyze the proteome of Escherichia coli strain K12 (JM109) and identified 1,480 expressed proteins, which are equivalent to 35 % of the total open reading frames predicted in the genome. This subset contained proteins with cellular abundance of several dozens to hundreds of thousands of copies, and included nearly all types of proteins in terms of chemical characteristics, subcellular distribution, and function. Interestingly, the subset also contained 138 of 164 gene products that are currently known to be essential for bacterial viability (84