Identification and Disruption of BetL, a Secondary Glycine Betaine Transport System Linked to the Salt Tolerance of Listeria monocytogenes LO28

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Abstract
The trimethylammonium compound glycine betaine (N,N,N-trimethylglycine) can be accumulated to high intracellular concentrations, conferring enhanced osmo- and cryotolerance upon Listeria monocytogenes. We report the identification of betL, a gene encoding a glycine betaine uptake system in L. monocytogenes, isolated by functional complementation of the betaine uptake mutantEscherichia coli MKH13. The betL gene is preceded by a consensus ςB-dependent promoter and is predicted to encode a 55-kDa protein (507 amino acid residues) with 12 transmembrane regions. BetL exhibits significant sequence homologies to other glycine betaine transporters, including OpuD fromBacillus subtilis (57% identity) and BetP fromCorynebacterium glutamicum (41% identity). These high-affinity secondary transporters form a subset of the trimethylammonium transporter family specific for glycine betaine, whose substrates possess a fully methylated quaternary ammonium group. The observed Km value of 7.9 μM for glycine betaine uptake after heterologous expression of betL inE. coli MKH13 is consistent with values obtained forL. monocytogenes in other studies. In addition, abetL knockout mutant which is significantly affected in its ability to accumulate glycine betaine in the presence or absence of NaCl has been constructed in L. monocytogenes. This mutant is also unable to withstand concentrations of salt as high as can the BetL+ parent, signifying the role of the transporter in Listeria osmotolerance.