In prokaryotes, genomic distance is a feature that in addition to coregulation affects coexpression.
Several observations, such as genomic clustering of highly coexpressed small
regulons, support the idea that coexpression behavior of coregulated genes is affected by
the distance between the coregulated genes. However, the specific contribution of distance
in addition to coregulation in determining the degree of coexpression has not yet been studied
systematically. In this work, we exploit the rich information in RegulonDB to study how
the genomic distance between coregulated genes affects their degree of coexpression,
measured by pairwise similarity of expression profiles obtained under a large number of conditions.
We observed that, in general, coregulated genes display higher degrees of coexpression
as they are more closely located on the genome. This contribution of genomic
distance in determining the degree of coexpression was relatively small compared to the
degree of coexpression that was determined by the tightness of the coregulation (degree of
overlap of regulatory programs) but was shown to be evolutionary constrained. In addition,
the distance effect was sufficient to guarantee coexpression of coregulated genes that are
located at very short distances, irrespective of their tightness of coregulation. This is partly
but definitely not always because the close distance is also the cause of the coregulation. In
cases where it is not, we hypothesize that the effect of the distance on coexpression could
be caused by the fact that coregulated genes closely located to each other are also relatively
more equidistantly located from their common TF and therefore subject to more similar levels
of TF molecules. The absolute genomic distance of the coregulated genes to their common
TF-coding gene tends to be less important in determining the degree of coexpression.
Our results pinpoint the importance of taking into account the combined effect of distance and coregulation when studying prokaryotic coexpression and transcriptional regulation.