Transcription unit conservation in the three domains of life: a perspective from Escherichia coli

Here we address the question of the degree to which genes within experiment ally characterized operons in one organism (Escherichia coli) are conserved in other genomes. We found that two genes adjacent within an operon are mo re likely both to have an ortholog in other organisms, regardless of relati ve position, than genes adjacent on the same strand but in two different tr anscription units. They are also more likely to occur next to, or fused to, one another in other genomes. Genes frequently conserved adjacent to each other, especially among evolutionarily distant species, must be part of the same transcription unit in most of them.