Physiological roles of an Acinetobacter-specific σ factor

Area of Science:

• Microbiology
• Bacterial Physiology
• Regulatory Networks

Background:

• Acinetobacter baumannii is an urgent threat due to antibiotic resistance.
• Understanding stress response mechanisms in A. baumannii can reveal therapeutic targets.
• Extracytoplasmic function (ECF) sigma factors, like RpoE, manage cellular stress in bacteria.

Purpose of the Study:

• To investigate the role of the poorly characterized ECF SigAb in Acinetobacter species.
• To determine if SigAb functions similarly to RpoE in stress mitigation.
• To define the SigAb regulon and its physiological impact in A. baumannii.

Main Methods:

• Promoter mutagenesis and motif scanning to define the direct SigAb regulon.
• Chromatin immunoprecipitation-sequencing (ChIP-seq) to identify SigAb binding sites.
• RNA-sequencing (RNA-seq) to analyze the indirect SigAb regulon.
• CRISPRi knockdown and Tn-seq for gene fitness assessment.

Main Results:

• SigAb is a metal stress-responsive ECF unique to Acinetobacter, distinct from RpoE.
• The direct SigAb regulon includes genes for SigAb, RelA, and the small RNA SabS.
• SigAb indirectly regulates hundreds of genes, including metal resistance genes, impacting cell physiology.
• Genes aabA and aabB within the sigAb operon exhibit anti-SigAb activity.
• SigAb, aabA, and aabB are essential for bacterial fitness even under optimal growth conditions.

Conclusions:

• SigAb plays a critical role in Acinetobacter's response to metal stress, particularly copper.
• SigAb is essential for bacterial fitness and survival, highlighting its importance in A. baumannii.
• The findings reveal a distinct regulatory architecture in A. baumannii and potential therapeutic targets.