Contribution of mechanosensitive channels to osmoadaptation and ectoine excretion in Halomonas elongata

Abstract

For osmoadaptation the halophilic bacterium Halomonas elongata synthesizes as its main compatible solute the aspartate derivative ectoine. H. elongata does not rely entirely on synthesis but can accumulate ectoine by uptake from the surrounding environment with the help of the osmoregulated transporter TeaABC. Disruption of the TeaABC-mediated ectoine uptake creates a strain that is constantly losing ectoine to the medium. However, the efflux mechanism of ectoine in H. elongata is not yet understood. H. elongata possesses four genes encoding mechanosensitive channels all of which belong to the small conductance type (MscS). Analysis by qRT-PCR revealed a reduction in transcription of the mscS genes with increasing salinity. The response of H. elongata to hypo-and hyperosmotic shock never resulted in up-regulation but rather in downregulation of mscS transcription. Deletion of all four mscS genes created a mutant that was unable to cope with hypoosmotic shock. However, the knockout mutant grew significantly faster than the wildtype at high salinity of 2 M NaCl, and most importantly, still exported 80% of the ectoine compared to the wildtype. We thus conclude that a yet unknown system, which is independent of mechanosensitive channels, is the major export route for ectoine in H. elongata.

Microbial production of ectoine and hydroxyectoine as high-value chemicals

“…By adopting the “bacterial milking” process, a final ectoine titer of 7.4 g/L was achieved after repeatedly performing this process at least nine times, with a productivity of 0.22 g/L/h. Mechanosensitive channels (MSC), which are responsible for the excretion of compatible solutes in many halophilic microorganisms when suffering hypoosmotic shock, only make a small contribution to the ectoine excretion in H. elongata [ 37 , 38 ]. Therefore, H. elongata should have an efflux system either as a universal compatible solute transporter or as a specific ectoine exporter [ 38 ].…”

The ups and downs of ectoine: structural enzymology of a major microbial stress protectant and versatile nutrient

“…Compatible solutes are released into the environment through the transient opening of mechanosensitive channels when producer cells are subjected to a rapid osmotic down-shift (Booth 2014). Microorganisms also excrete compatible solutes under steady-state high osmolarity growth conditions, perhaps in an effort to finetune turgor (Czech et al 2016(Czech et al , 2018bGrammann et al 2002;Hoffmann et al 2012;Lamark et al 1992;Vandrich et al 2020). An ecophysiologically important process for the release of compatible solutes into the environment is also cell lysis; e.g., after the attack of ectoine producers by lytic phages (Van Goethem et al 2019).…”

“…EhuB and UehA, the periplasmic high-affinity substrate-binding proteins have been crystalized in the presence of ectoines, thereby revealing the molecular determinants for the efficient capturing of environmental ectoines by transporters (Hanekop et al 2007;Lecher et al 2009) (Supplementary Figure S6). The osmotically inducible TeaABC transporter form H. elongata, a system closely related to the TRAP-T UehABC system from Ruegeria pomeroyi, not only serves for the import of ectoines as osmostress protectants and as a recycling systems for newly synthesized and excreted ectoines (Grammann et al 2002;Kuhlmann et al 2008b;Vandrich et al 2020) but it might also be used for their acquisition as nutrients (Schwibbert et al 2011).…”

“…Shown are the systems that mediate cellular import of K + under acute osmotic stress as an immediate adjustment response and those that allow it extrusion once the cell begins with the import and/or synthesis of compatible solutes (e.g., ectoine and its derivative 5-hydroxyectoine) (Bremer and Krämer 2019;Wood 2011). Cells also seem to possess efflux systems for compatible solutes, probably a measure to fine-tune turgor when the cell elongates and doubles its volume prior to division (Czech et al 2016;Vandrich et al 2020). Cells maintain very low intracellular Na + concentration through dedicated export systems as this ion, in contrast to K + , is cytotoxic (Danchin and Nikel 2019).…”

Extremophilic Microorganisms for the Treatment of Toxic Pollutants in the Environment

“…For example, the halophilic bacterium Spiribacter salinus M19-40 produces enhanced levels of compatible solutes such as ectoine and trehalose when they are exposed to a high NaCl concentration [ 45 ]. These organic solutes have a critical role in reducing the thermodynamic activity of water to compensate for the external osmotic pressure [ 47 ].…”