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A fundamental principle of exponential bacterial growth is that no more ribosomes are produced than are necessary to support the balance between nutrient availability and protein synthesis. Although this conclusion was first expressed more than 40 years ago, a full understanding of the molecular mechanisms involved remains elusive and the issue is still controversial. There is currently agreement that, although many different systems are undoubtedly involved in fine-tuning this balance, an important control, and in our opinion perhaps the main control, is regulation of the rate of transcription initiation of the stable (ribosomal and transfer) RNA transcriptons. In this review, we argue that regulation of DNA supercoiling provides a coherent explanation for the main modes of transcriptional control - stringent control, growth-rate control and growth-phase control - during the normal growth of Escherichia coli.

Regulation of cellular growth implies spatiotemporally coordinated programmes of gene transcription. A central question, therefore, is how global transcription is coordinated in the genome. The growth of the unicellular organism Escherichia coli is associated with changes in both the global superhelicity modulated by cellular topoisomerase activity and the relative proportions of the abundant DNA-architectural chromatin proteins. Using a DNA-microarray-based approach that combines mutations in the genes of two important chromatin proteins with induced changes of DNA superhelicity, we demonstrate that genomic transcription is tightly associated with the spatial distribution of supercoiling sensitivity, which in turn depends on chromatin proteins. We further demonstrate that essential metabolic pathways involved in the maintenance of growth respond distinctly to changes of superhelicity. We infer that a homeostatic mechanism organizing the supercoiling sensitivity is coordinating the growth-phase-dependent transcription of the genome.

In Escherichia coli crosstalk between DNA supercoiling, nucleoid-associated proteins and major RNA polymerase σ initiation factors regulates growth phase-dependent gene transcription. We show that the highly conserved spatial ordering of relevant genes along the chromosomal replichores largely corresponds both to their temporal expression patterns during growth and to an inferred gradient of DNA superhelical density from the origin to the terminus. Genes implicated in similar functions are related mainly in trans across the chromosomal replichores, whereas DNA-binding transcriptional regulators interact predominantly with targets in cis along the replichores. We also demonstrate that macrodomains (the individual structural partitions of the chromosome) are regulated differently. We infer that spatial and temporal variation of DNA superhelicity during the growth cycle coordinates oxygen and nutrient availability with global chromosome structure, thus providing a mechanistic insight into how the organization of a complete bacterial chromosome encodes a spatiotemporal program integrating DNA replication and global gene expression. In Escherichia coli cells the physiological transitions induced by the changing growth environment are accompanied by changes in DNA superhelical density (1-3), nucleoid structure (4-6), and the promoter selectivity of the RNA polymerase (RNAP) holoenzyme (3, 7). During the growth cycle both the relative and absolute concentrations of the abundant nucleoid-associated proteins (NAPs ; Table S1) change substantially and correspondingly generate bacterial chromatin of variable composition (8, 9). The NAPs stabilize distinct supercoil structures (10-12) selectively favoring particular RNAP holoenzymes (13-15). These variable nucleoprotein complexes modulate DNA topology during the growth cycle (Fig. 1A), optimizing the channeling of supercoil energy into appropriate metabolic pathways (16,17).The expression of the genes determining superhelical density, polymerase selectivity, and nucleoid structure is coordinated by cross-regulation. Thus, factor for inversion stimulation (FIS), a NAP abundant during the early exponential phase (18), regulates expression not only of the superhelicity determinants DNA gyrase subunits A and B (gyrA and gyrB) and topoisomerase I (topA) (19-21) but also other NAP-encoding genes including hns, α subunit of histone-like protein from E. coli strain U93 (hupA), and DNA binding protein from starved cells (dps) (22-24) and components of the transcription machinery such as σ 38 subunit of RNA polymerase rpoS (25). Similarly mutations affecting the selectivity of RNAP influence NAP production (26-28). Again, mutations in the genes controlling DNA superhelicity affect the production of both NAPs and the basal transcription machinery (27,29). This pattern of integrated control constitutes a heterarchical network coordinating chromosome structure with cellular metabolism (27,28,30). A further pointer to this integrated network is the observed selection of mutations in fis...