A Two-Component System That Modulates Cyclic di-GMP Metabolism Promotes Legionella pneumophila Differentiation and Viability in Low-Nutrient Conditions.

During its life cycle, the environmental pathogen Legionella pneumophila alternates between a replicative and transmissive cell type when cultured in broth, macrophages, or amoebae. Within a protozoan host, L. pneumophila further differentiates into the hardy cell type known as the mature infectious form (MIF). The second messenger cyclic di-GMP coordinates lifestyle changes in many bacterial species, but its role in the L. pneumophila life cycle is less understood. Using an in vitro broth culture model that approximates the intracellular transition from the replicative to the transmissive form, here we investigate the contribution to L. pneumophila differentiation of a two-component system (TCS) that regulates cyclic di-GMP metabolism. The TCS is encoded by lpg0278-lpg0277 and is cotranscribed with lpg0279, which encodes a protein upregulated in MIF cells. The promoter for this operon is RpoS dependent and induced in nutrient-limiting conditions that do not support replication, as demonstrated using a gfp reporter and quantitative PCR (qPCR). The response regulator of the TCS (Lpg0277) is a bifunctional enzyme that both synthesizes and degrades cyclic di-GMP. Using a panel of site-directed point mutants, we show that cyclic di-GMP synthesis mediated by a conserved GGDEF domain promotes growth arrest of replicative L. pneumophila, accumulation of pigment and poly-3-hydroxybutyrate storage granules, and viability in nutrient-limiting conditions. Genetic epistasis tests predict that the MIF protein Lpg0279 acts as a negative regulator of the TCS. Thus, L. pneumophila is equipped with a regulatory network in which cyclic di-GMP stimulates the switch from a replicative to a resilient state equipped to survive in low-nutrient environments.IMPORTANCE Although an intracellular pathogen, L. pneumophila has developed mechanisms to ensure long-term survival in low-nutrient aqueous conditions. Eradication of L. pneumophila from contaminated water supplies has proven challenging, as outbreaks have been traced to previously remediated systems. Understanding the genetic determinants that support L. pneumophila persistence in low-nutrient environments can inform design and assessment of remediation strategies. Here we characterize a genetic locus that encodes a two-component signaling system (lpg0278-lpg0277) and a putative regulator protein (lpg0279) that modulates the production of the messenger molecule cyclic di-GMP. We show that this locus promotes both L. pneumophila cell differentiation and survival in nutrient-limiting conditions, thus advancing the understanding of the mechanisms that contribute to L. pneumophila environmental resilience.

Molecular response to the pathogen Phytophthora sojae among ten soybean near isogenic lines revealed by comparative transcriptomics.

BACKGROUND: Phytophthora root and stem rot (PRR) of soybean, caused by Phytophthora sojae, is controlled by Rps genes. However, little is known regarding the Rps-induced molecular responses to P. sojae and how they actually overlap. We thus sequenced, analyzed, and compared the transcriptomes of 10 near isogenic lines (NILs), each with a unique Rps gene/allele, and the susceptible parent Williams, pre- and post-inoculation with the pathogen. RESULTS: A total of 4,330 differentially expressed genes (DEGs) were identified in Williams versus 2,014 to 5,499 DEGs in individual NILs upon inoculation with the pathogen. Comparisons of the DEGs between the NILs and Williams identified incompatible interaction genes (IIGs) and compatible interaction genes (CIGs). Hierarchical cluster and heatmap analyses consistently grouped the NILs into three clusters: Cluster I (Rps1-a), Cluster II (Rps1-b, 1-c and 1-k) and Cluster III (Rps3-a, 3-b, 3-c, 4, 5, and 6), suggesting an overlap in Rps-induced defense signaling among certain NILs. Gene ontology (GO) analysis revealed associations between members of the WRKY family and incompatible reactions and between a number of phytohormone signaling pathways and incompatible/compatible interactions. These associations appear to be distinguished according to the NIL clusters. CONCLUSIONS: This study characterized genes and multiple branches of putative regulatory networks associated with resistance to P. sojae in ten soybean NILs, and depicted functional "fingerprints" of individual Rps-mediated resistance responses through comparative transcriptomic analysis. Of particular interest are dramatic variations of detected DEGs, putatively involved in ethylene (ET)-, jasmonic acid (JA)-, (reactive oxygen species) ROS-, and (MAP-kinase) MAPK- signaling, among these soybean NILs, implicating their important roles of these signaling in differentiating molecular defense responses. We hypothesize that different timing and robustness in defense signaling to the same pathogen may be largely responsible for such variations.

How Legionella defend their turf.

Communities of bacteria that cause Legionnaires' disease repel other bacteria by secreting an acid called HGA.