Genes that interact or function together are often clustered in bacterial genomes, and it has been proposed that this clustering may affect gene expression. In this study, we directly compared gene expression in nonclustered arrangements and in three common clustered arrangements (codirectional, divergent, and operon) using synthetic circuits in Escherichia coli. We found that gene clustering had minimal effects on gene expression. Specifically, gene clustering did not alter constitutive expression levels or stochastic fluctuations in expression ("expression noise"). Remarkably, the expression of two genes that share the same chromosome position with the same promoter (operon) or with separate promoters (codirectional and divergent arrangements) was not significantly more correlated than genes at different chromosome positions (nonclustered arrangements). The only observed effect of clustering was increased transcription factor binding in codirectional and divergent gene arrangements due to DNA looping, but this is not a specific feature of clustering. In summary, we demonstrate that gene clustering is not a general modulator of gene expression, and therefore any effects of clustering are likely to occur only with specific genes or under certain conditions.
BACTERIAL genes that have products that interact or function within a common pathway are often adjacent to one another on the chromosome (i.e., the genes are "clustered") (reviewed in Rocha 2008). Genes that are commonly clustered include those that encode proteins in a metabolic pathway or proteins in multi-protein complexes. Regulatory genes that encode transcription factors are also commonly located next to their target genes. Common arrangements of clustered genes are (1) codirectional (//, genes on the same DNA strand with separate promoters); (2) divergent ()/, genes on opposite DNA strands with separate promoters); and (3) operons (/, genes on the same DNA strand with a shared promoter) (Korbel et al. 2004). Clustered genes that are located on opposite DNA strands with convergent transcription (/)) are less common.A common functional explanation for gene clustering is that it results in more correlated gene expression. That is, genes that are next to each other on the chromosome are more likely to be affected in the same way by DNA compaction and supercoiling (Jeong et al. 2004;Kepes 2004;Peter et al. 2004;Allen et al. 2006;Mathelier and Carbone 2010), gene dosage (Cooper and Helmstetter 1968;Chandler and Pritchard 1975;Schmid and Roth 1987;Sousa et al. 1997), and neighboring sequences (De and Babu 2010). Therefore, the expression of clustered genes is expected to be more likely to increase and decrease in synchrony (due to variation in these local chromosome position effects), resulting in more correlated expression than occurs with nonclustered genes. It has been proposed that more correlated gene expression helps maintain the optimal stoichiometry of the gene products, thereby increasing efficiency and/or decreasing the concentration of tox...