A Dual Role for the Bacillus anthracis Master Virulence Regulator AtxA: Control of Sporulation and Anthrax Toxin Production

Abstract

Bacillus anthracis is an endemic soil bacterium that exhibits two different lifestyles. In the soil environment, B. anthracis undergoes a cycle of saprophytic growth, sporulation, and germination. In mammalian hosts, the pathogenic lifestyle of B. anthracis is spore germination followed by vegetative cell replication, but cells do not sporulate. During infection, and in specific culture conditions, transcription of the structural genes for the anthrax toxin proteins and the biosynthetic operon for capsule synthesis is positively controlled by the regulatory protein AtxA. A critical role for the atxA gene in B. anthracis virulence has been established. Here we report an inverse relationship between toxin production and sporulation that is linked to AtxA levels. During culture in conditions favoring sporulation, B. anthracis produces little to no AtxA. When B. anthracis is cultured in conditions favoring toxin gene expression, AtxA is expressed at relatively high levels and sporulation rate and efficiency are reduced. We found that a mutation within the atxA promoter region resulting in AtxA over-expression leads to a marked sporulation defect. The sporulation phenotype of the mutant is dependent upon pXO2-0075, an atxA-regulated open reading frame located on virulence plasmid pXO2. The predicted amino acid sequence of the pXO2-0075 protein has similarity to the sensor domain of sporulation sensor histidine kinases. It was shown previously that pXO2-0075 overexpression suppresses sporulation. We have designated pXO2-0075 “skiA” for “sporulation kinase inhibitor.” Our results indicate that in addition to serving as a positive regulator of virulence gene expression, AtxA modulates B. anthracis development.

A mobile genetic element increases bacterial host fitness by manipulating development

“…Two plasmids, pXO1 and pXO2, provide the genes for toxin synthesis and production of a protective capsule, respectively (Green et al ., 1985; Mikesell et al ., 1983). pXO1 also contains a regulatory gene, atxA , that regulates virulence factor production and inhibits host cell sporulation (Dale et al ., 2018). Co-regulation of virulence factors and sporulation is likely important during infection, as B. anthracis spores are thought to be more susceptible than vegetative cells to eradication by the immune system (Mock and Fouet, 2001).…”

Spo0A Suppresses sin Locus Expression in Clostridioides difficile

“…When the post-exponential phase begins, Spo0A activates the expression of the genes involved in the sporulation initiation process and positively regulates the sigma factor cascade required for sporulation (29). In many other pathogenic spore-forming bacteria, the gene regulatory networks that influence sporulation and virulence are closely linked with each other (42)(43)(44)(45)(46). In C. difficile, the mutation in spo0A affected many pathogenic traits, including toxin production, flagellum expression, and biofilm formation (5,28,30,31,47).…”

Spo0A suppressessinlocus expression inClostridioides difficile

“…When the post-exponential phase begins, Spo0A activates the expression of the genes involved in the sporulation initiation process and positively regulates the sigma factor cascade required for sporulation (38). In many other pathogenic spore-forming bacteria the gene regulatory networks that influence sporulation and virulence are closely linked with each other (49–53). In C. difficile , the mutation in spo0A affected many pathogenic traits, including toxin production, flagella expression and biofilm formation (5, 32, 33, 39).…”