  We have performed a statistical analysis of the spatial distribution of
operons in the transcriptional regulation network of Escherichia coli. The
analysis reveals that operons that regulate each other and operons that are
coregulated tend to lie next to each other on the genome. Moreover, these pairs
of operons tend to be transcribed in diverging directions. This spatial
arrangement of operons allows the upstream regulatory regions to interfere with
each other. This affords additional regulatory control, as illustrated by a
mean-field analysis of a feed-forward loop. Our results suggest that regulatory
control can provide a selection pressure that drives operons together in the
course of evolution.
