# Bacterial chromatin remodeling associated with transcription-induced domains at pathogenicity Islands

## TL;DR

This study shows how Salmonella bacteria regulate harmful genes by restructuring their DNA and positioning them near the cell's edge when they become active.

## Contribution

The study reveals how chromatin remodeling and spatial repositioning regulate SPI-1 gene expression in Salmonella.

## Key findings

- Silent SPIs are linked to antisense transcription from H-NS-free regions.
- SPI-1 activation causes H-NS remodeling and forms transcription-induced domains.
- SPI-1 repositions to the nucleoid periphery when activated.

## Abstract

The nucleoid-associated protein H-NS is a bacterial xenogeneic silencer responsible for preventing costly expression of genes acquired through horizontal gene transfer. H-NS silences several Salmonella Pathogenicity Islands (SPIs) essential for host infection. The stochastic expression of SPI-1 is required for invasion of host epithelial cells but complicates investigation of factors involved in SPI-1 chromatin structure and regulation. We performed functional genomics on sorted Salmonella populations expressing SPI-1 or not, to characterize how SPI-1 activation affects chromatin composition, DNA conformation, gene expression and SPI-1 subcellular localization. We show that silent SPIs are associated with spurious antisense transcriptional activity originating from H-NS-free regions. Upon SPI-1 activation, remodeling of H-NS occupancy defines a new chromatin landscape, which together with the master SPI-1 regulator HilD, facilitates transcription of SPI-1 genes. SPI-1 activation promotes formation of Transcription Induced Domains accompanied by repositioning SPI-1 close to the nucleoid periphery. We present a model for tightly regulated chromatin remodeling that minimizes the cost of pathogenicity island activation.

The bacterial protein H-NS prevents costly expression of horizontally acquired genes such as those in Salmonella pathogenicity islands (SPIs), which are essential for infection. Here, Kortebi et al. show that the expression of SPI-1 is associated with Salmonella chromatin remodelling and with the repositioning of this region toward the nucleoid periphery.