Transcriptional Regulation and Posttranslational Activity of the Betaine Transporter BetL in Listeria monocytogenes Are Controlled by Environmental Salinity

While the genetic elements contributing to the salinity tolerance of Listeria monocytogenes have been well characterized, the regulatory signals and responses (genetic and/or biochemical) that govern these mechanisms have yet to be elucidated. Encoded by betL, the first genetic element to be linked to listerial osmotolerance, the secondary betaine uptake system BetL is a member of the betaine-carnitine-choline transporter family. Preceded by consensus σ^A- and σ^B-dependent promoter sites, betL is constitutively expressed and transcriptionally up-regulated in response to salt stress. The nisin-controlled expression system was used to achieve salinity-independent, controlled betL expression in Listeria. In the absence of NaCl-activated transcriptional control, BetL activity was found to be a function of environmental salinity, showing optimal activity in buffer supplemented with 1 to 2% NaCl (osmolality, 417 to 719 mosmol/kg). In addition, BetL was activated rapidly (half-life, 2 min) in response to an osmotic upshift imposed by adding 2% NaCl to 50 mM potassium phosphate buffer.