Physiological roles of an Acinetobacter -specific σ factor

Journal Article · 18 May 2025 · mBio (Online)

DOI: https://doi.org/10.1128/mbio.00968-25 · OSTI ID:2567096

Bacon, Emily E. ; Myers, Kevin S. ; Iruegas-López, Rubén; Banta, Amy B. ; Place, Michael; Ebersberger, Ingo; Peters, Jason M. ; Trent, ed., M. Stephen

ABSTRACT <p> The Gram-negative pathogen <italic toggle='yes'>Acinetobacter baumannii</italic> is considered an “urgent threat” to human health due to its propensity to become antibiotic resistant. Understanding the distinct regulatory paradigms used by <italic toggle='yes'>A. baumannii</italic> to mitigate cellular stresses may uncover new therapeutic targets. Many γ-proteobacteria use the extracytoplasmic function (ECF) σ factor, RpoE, to invoke envelope homeostasis networks in response to stress. <italic toggle='yes'>Acinetobacter</italic> species contain the poorly characterized ECF “SigAb”; however, it is unclear if SigAb has the same physiological role as RpoE. Here, we show that SigAb is a metal stress-responsive ECF that appears unique to <italic toggle='yes'>Acinetobacter</italic> species and distinct from RpoE-like ECFs. We combine promoter mutagenesis, motif scanning, and chromatin immunoprecipitation-sequencing (ChIP-seq) to define the direct SigAb regulon, which consists of genes encoding SigAb itself, the stringent response mediator, RelA, and the uncharacterized small RNA, “SabS.” However, RNA-seq of strains overexpressing SigAb revealed a large, indirect regulon containing hundreds of genes. Metal resistance genes are key elements of the indirect regulon, as CRISPRi knockdown of <italic toggle='yes'>sigAb</italic> or <italic toggle='yes'>sabS</italic> resulted in increased copper sensitivity and excess copper-induced SigAb-dependent transcription. Furthermore, we found that two uncharacterized genes in the <italic toggle='yes'>sigAb</italic> operon, “ <italic toggle='yes'>aabA</italic> ” and “ <italic toggle='yes'>aabB</italic> ,” have anti-SigAb activity. Finally, employing a targeted Tn-seq approach that uses CRISPR-associated transposons, we show that <italic toggle='yes'>sigAb</italic> , <italic toggle='yes'>aabA</italic> , and <italic toggle='yes'>aabB</italic> are important for fitness even during optimal growth conditions. Our work reveals new physiological roles for SigAb and SabS, provides a novel approach for assessing gene fitness, and highlights the distinct regulatory architecture of <italic toggle='yes'>A. baumannii</italic> . </p> </sec> <sec> <title>IMPORTANCE Acinetobacter baumannii is a hospital-acquired pathogen, and many strains are resistant to multiple antibiotics. Understanding how A. baumannii senses and responds to stress may uncover novel routes to treat infections. Here, we examine how the Acinetobacter -specific transcription factor, SigAb, mitigates stress. We find that SigAb directly regulates only a small number of genes, but indirectly controls hundreds of genes that have substantial impacts on cell physiology. We show that SigAb is required for maximal growth, even during optimal conditions, and is acutely required during growth in the presence of elevated copper. Given that copper toxicity plays roles in pathogenesis and on copper-containing surfaces in hospitals, we speculate that SigAb function may be important in clinically relevant contexts.

• Sponsoring Organization: USDOE
• Grant/Contract Number: SC0018409
• OSTI ID: 2567096
• Journal Information: mBio (Online), Journal Name: mBio (Online); ISSN 2150-7511
• Publisher: American Society for MicrobiologyCopyright Statement
• Country of Publication: United States
• Language: English