  The distribution of most genes is not random, and functionally linked genes
are often found in clusters. Several theories have been put forward to explain
the emergence and persistence of operons in bacteria. Careful analysis of
genomic data favours the co-regulation model, where gene organization into
operons is driven by the benefits of coordinated gene expression and
regulation. Direct evidence that co-expression increases the individual's
fitness enough to ensure operon formation and maintenance is, however, still
lacking. Here, a previously described quantitative model of the network that
controls the transcription factor sigmaF during sporulation in Bacillus
subtilis is employed to quantify the benefits arising from both organisation of
the sporulation genes into the spoIIA operon and from translational coupling.
The analysis shows that operon organization, together with translational
coupling, is important because of the inherent stochastic nature of gene
expression which skews the ratios between protein concentrations in the absence
of co- regulation. The predicted impact of different forms of gene regulation
on fitness and survival agrees quantitatively with published sporulation
efficiencies.
