Glycine Betaine Transport in Lactococcus lactis Is Osmotically Regulated at the Level of Expression and Translocation Activity

Abstract

Microorganisms react upon hyperosmotic stress by accumulating compatible solutes. Here we report that Lactococcus lactis uses a transport system for glycine betaine that, contrary to earlier observations (D. Molenaar et al., J. Bacteriol. 175:5438-5444, 1993), is osmotically regulated at the levels of both expression and transport activity.In their natural habitats, microorganisms are often exposed to changes in the concentrations of the solutes in their environment whereas the internal concentrations of nutrients need to be relatively constant (2,8,13). A sudden increase in the osmolarity of the environment results in the movement of water from the cell to the outside medium, which causes turgor pressure loss, intracellular solute concentration changes, and cell volume changes. Such hyperosmotic conditions are detrimental to any living cell. Bacteria counteract hyperosmotic stress by accumulating compatible solutes by uptake and/or synthesis. These solutes can be accumulated to high intracellular concentrations without affecting vital cellular processes, and they restore the osmotic balance of the cell. Upon hypoosmotic stress, these compatible solutes are released from the cell, which prevents too high a turgor pressure that may ultimately lead to bursting of the cell.Lactic acid bacteria have a limited capacity to synthesize compatible solutes, and a range of studies indicate that glycine betaine, carnitine, and proline are the most important compatible solutes in this group of organisms (3,4,11,12,19). The role of these compounds in osmoregulation in other (micro)-organisms is also well established (2,5,9,13,15,16), but in those cases, additional molecules play a major role as well. There is a considerable amount of data that indicate that in Lactobacillus plantarum and Listeria monocytogenes, glycine betaine, carnitine, and proline are taken up via semiconstitutive transport systems that are activated upon hyperosmotic stress, whereas these compounds are rapidly released by channel-like activities upon osmotic downshock (3, 19). These studies prompted us to reinvestigate the regulation of glycine betaine transport in Lactococcus lactis, which is thought not to respond to any form of osmotic stress whereas the pool sizes during growth in different media do (11). Osmotic regulation of transport through alterations in activity has not only been shown in lactic acid bacteria but is also well documented for other bacteria (1,10,13,15). In this study, we examined the regulation of glycine betaine uptake in two well-defined L. lactis strains that in many respects are paradigmatic of our knowledge of the physiology, energetics, and genetics of lactic acid bacteria. L. lactis MG1363 is a plasmid-free derivative of ML3 that was studied previously (11), whereas IL1403 is a plasmid-free strain for which the genome sequence will soon become available.Regulation of glycine betaine uptake by osmotic upshock. Cells were grown in CDM (14) (with or without proline) plus 25 mM glucose and 500 mM KCl, harvested by centrifugati...

Lactococcus lactis Uses MscL as Its Principal Mechanosensitive Channel

“…After 15 PCR cycles, it is clearly visible that the transcription of opuAA (the first gene of the opuA operon) was increased in the cells grown at high salt (Fig. 5A, compare lane A3 with lane B3), which is in agreement with a higher glycine betaine uptake capacity under these conditions (13). RT-PCR DNA corresponding to mscL and yncB fragments were visible after 22 PCR cycles.…”

“…The electrophysiological characteristics of MscS from E. coli (12) and of a number of MscL homologues (5) have been described. So far, their physiological roles in osmoprotection are not fully understood.Previously, we have established the mechanism of the osmotic regulation of the upshift-activated glycine betaine transporter OpuA from Lactococcus lactis (13, 14). Although the physicochemical properties of the membrane and lipid-protein interactions also play a critical role in the osmotic activation of OpuA, the mechanism is entirely different from that underlying the gating of MS 1 channels.…”

Conformational study of sarcosine as probed by matrix-isolation FT-IR spectroscopy and molecular orbital calculations

“…These molecules are known to affect the melting behavior of proteins and nucleic acids, and are also known as osmoprotectants or osmolytes [1][2][3][4]. They are produced in organisms and cellular systems that have adapted to extreme environmental conditions, such as high temperatures, very low humidity and high salinity [1,2].…”

La réponse au stress osmotique des bactéries lactiques Lactococcus lactis et Lactobacillus plantarum (mini-revue)

“…Une augmentation de l'activité de transport de la glycine bétaïne via QacT a été observée spécifiquement lors d'une élévation de la concentration en sel du milieu extérieur et constituerait, pour L. plantarum, la réponse initiale à un choc hypertonique. Des études chez L. lactis ont aussi démontré l'existence d'un opéron codant un système de transport à haute affinité pour la glycine bétaïne appelé BusA (betaine uptake system A) ou OpuA (osmoprotectant uptake A), dont l'expression augmente spécifiquement lors d'un choc hypertonique [2,27,36]. OpuA de L. lactis appartient à la famille des transporteurs de type ABC (ATP Binding Cassette).…”

“…La régulation osmotique d'opuA semble principalement porter sur l'expression de l'opéron, même si des données suggèrent aussi l'existence d'un contrôle au niveau de l'activité du transporteur [36]. La poursuite de l'étude de la régulation osmotique de opuA par OpuR permettra une meilleure compréhension des mécanismes molécu-laires de la réponse à un choc osmotique chez L. lactis.…”