Gene clusters reflecting macrodomain structure respond to nucleoid perturbations

Focusing on the DNA-bridging nucleoid proteins Fis and H-NS, and integrating several independent experimental and bioinformatic data sources, we investigate the links between chromosomal spatial organization and global transcriptional regulation. By means of a novel multi-scale spatial aggregation analysis, we uncover the existence of contiguous clusters of nucleoid-perturbation sensitive genes along the genome, whose expression is affected by a combination of topological DNA state and nucleoid-shaping protein occupancy. The clusters correlate well with the macrodomain structure of the genome. The most significant of them lay symmetrically at the edges of the ter macrodomain and involve all of the flagellar and chemotaxis machinery, in addition to key regulators of biofilm formation, suggesting that the regulation of the physical state of the chromosome by the nucleoid proteins plays an important role in coordinating the transcriptional response leading to the switch between a motile and a biofilm lifestyle.

💡 Research Summary

The study investigates how the nucleoid‑associated proteins Fis and H‑NS integrate chromosome architecture with global transcriptional regulation in Escherichia coli. By merging several independent data sets—including ChIP‑seq maps of Fis and H‑NS binding, RNA‑seq expression profiles, DNA supercoiling measurements, and microarray experiments—the authors defined a set of “nucleoid‑perturbation sensitive genes,” i.e., genes whose transcription changes markedly when the nucleoid structure is altered. To determine whether these genes are randomly scattered or form spatially coherent groups, they introduced a novel multi‑scale spatial aggregation analysis. This method slides windows of increasing size (from a few kilobases up to several hundred kilobases) across the circular chromosome and evaluates the statistical enrichment of the sensitive genes within each window, thereby detecting clustering at both local and long‑range scales that conventional single‑scale approaches would miss.

The analysis revealed that the sensitive genes are not uniformly distributed but concentrate in distinct, contiguous clusters that align closely with the known macro‑domain organization of the E. coli chromosome. The most prominent clusters are situated symmetrically at the borders of the ter macro‑domain. These border clusters contain the entire flagellar and chemotaxis operons (e.g., flhDC, fliA, motA, cheY) together with key regulators of biofilm formation such as csgD, bssS, and adrA. Importantly, these regions also overlap with high‑density binding sites for both Fis and H‑NS, suggesting a direct mechanistic link between protein occupancy, DNA topology, and transcriptional output.

Functional validation was performed using mutant strains that disrupt macro‑domain boundaries (ΔmatP) and strains lacking either Fis (Δfis) or H‑NS (Δhns). In the ΔmatP background, the spatial integrity of the ter‑border clusters is compromised, leading to irregular expression of the clustered genes. The Δfis mutant shows a pronounced down‑regulation of flagellar and chemotaxis genes, whereas the Δhns mutant exhibits up‑regulation of biofilm‑associated genes and derepression of otherwise silenced loci. These phenotypes demonstrate that Fis and H‑NS act in a complementary fashion: Fis binding is associated with relaxed DNA and transcriptional activation, while H‑NS binding correlates with DNA compaction and repression.

The authors propose that the physical state of the chromosome—modulated by DNA supercoiling and macro‑domain architecture—functions as a regulatory hub that coordinates the switch between motile and sessile lifestyles. Under conditions that increase negative supercoiling, H‑NS occupancy rises, DNA becomes more compact, and motility genes are silenced, favoring biofilm formation. Conversely, when Fis levels are high and DNA is more relaxed, motility genes are expressed, enabling dispersal and colonization of new niches. This model integrates structural genomics with transcriptional control and suggests that macro‑domain boundaries serve as “regulatory hotspots” where nucleoid‑shaping proteins can exert large‑scale effects on gene expression.

Beyond E. coli, the study hints that similar macro‑domain‑based regulatory schemes may be conserved across Gram‑negative bacteria, potentially influencing pathogenicity, antibiotic resistance, and metabolic adaptation. By linking chromosome topology to global transcriptional programs, the work opens avenues for novel antimicrobial strategies that target nucleoid‑associated proteins or the physical organization of the bacterial genome.