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Plant pathogenic bacteria are responsible for a substantial number of plant diseases worldwide, resulting in significant economic losses. Bacteria are exposed to numerous stress factors during their epiphytic life and within the host. Their ability to survive in the host and cause symptomatic infections depends on their capacity to overcome stressors. Bacteria have evolved a range of defensive and adaptive mechanisms to thrive under varying environmental conditions. One such mechanism involves the induction of chaperone proteins that belong to the heat shock protein (Hsp) family. Together with proteases, these proteins are integral components of the protein quality control system (PQCS), which is essential for maintaining cellular proteostasis. However, knowledge of their action is considerably less extensive than that of human and animal pathogens. This study discusses the modulation of Hsp levels by phytopathogenic bacteria in response to stress conditions, including elevated temperature, oxidative stress, changes in pH or osmolarity of the environment, and variable host conditions during infection. All these factors influence bacterial virulence. Finally, the secretion of GroEL and DnaK proteins outside the bacterial cell is considered a potentially important virulence trait. [ABSTRACT FROM AUTHOR]

High-temperature requirement A (HtrA; DegP) from Escherichia coli, an important element of the extracytoplasmic protein quality-control system, is a member of the evolutionarily conserved family of serine proteases. The characteristic feature of this protein is its allosteric mode of activation. The regulatory loops, L3, L2, L1 and LD, play a crucial role in the transmission of the allosteric signal. Yet, the role of LD has not been fully elucidated. Therefore, we undertook a study to explain the role of the individual LD residues in inducing and maintaining the proteolytic activity of HtrA. We investigated the influence of amino acid substitutions located within the LD loop on the kinetics of a model substrate cleavage as well as on the dynamics of the oligomeric structure of HtrA. We found that the mutations that were expected to disturb the loop's structure and/or interactions with the remaining regulatory loops severely diminished the proteolytic activity of HtrA. The opposite effect, that is, increased activity, was observed for G174S substitution, which was predicted to strengthen the interactions mediated by LD. Htr AG174S protein had an equilibrium shifted toward the active enzyme and formed preferentially high-order oligomeric forms. [ABSTRACT FROM AUTHOR]