Date Published: July 12, 2017
Publisher: Public Library of Science
Author(s): Bronwyn G. Butcher, Zhongmeng Bao, Janet Wilson, Paul Stodghill, Bryan Swingle, Melanie Filiatrault, David Schneider, Samuel Cartinhour, Eric Cascales.
The bacterial plant pathogen Pseudomonas syringae adapts to changes in the environment by modifying its gene expression profile. In many cases, the response is mediated by the activation of extracytoplasmic function (ECF) sigma factors that direct RNA polymerase to transcribe specific sets of genes. In this study we focus on PSPTO_1043, one of ten ECF sigma factors in P. syringae pv. tomato DC3000 (DC3000). PSPTO_1043, together with PSPTO_1042, encode an RpoERsp/ChrR-like sigma/anti-sigma factor pair. Although this gene pair is unique to the P. syringae group among the pseudomonads, homologous genes can be found in photosynthetic genera such as Rhodospirillum, Thalassospira, Phaeospirillum and Parvibaculum. Using ChIP-Seq, we detected 137 putative PSPTO_1043 binding sites and identified a likely promoter motif. We characterized 13 promoter candidates, six of which regulate genes that appear to be found only in P. syringae. PSPTO_1043 responds to the presence of singlet oxygen (1O2) and tert-butyl hydroperoxide (tBOOH) and several of the genes regulated by PSPTO_1043 appear to be involved in response to oxidative stress.
Pseudomonas syringae is a diverse species complex with more than 50 described pathovars causing speck, fleck, spot, blight and canker diseases on a wide range of hosts . However, individual isolates typically have a narrow host range. The organism is often present as an epiphyte but can enter the plant via stomata or wound sites, proliferate in intercellular spaces, and eventually cause disease. P. syringae pv. tomato DC3000 (DC3000)  causes bacterial speck on tomato plants and Arabidopsis thaliana and is a model organism for the study of plant pathogen interactions (reviewed in ).
In this report, we describe the ECF sigma factor, PSPTO_1043, which is predicted to be co-transcribed with PSPTO_1042, a likely anti-sigma factor. PSPTO_1043 and PSPTO_1042 are homologous to the RpoERsp-ChrR genes in Rhodobacter sphaeroides  and Azospirillum brasilense . The RpoERsp-ChrR system [14, 28] responses to the presence of 1O2, which is produced during photosynthesis, and may play an important role in protecting these photosynthetic bacteria from its harmful effects. A homologous system in Caulobacter crescentus, a free-living bacterium found in nutrient-poor aquatic environments, also responds to the presence of 1O2 and also cadmium .