CHARACTERISTICS AND OSMOREGULATORY ROLES OF UPTAKE SYSTEMS FOR PROLINE AND GLYCINE BETAINE IN LACTOCOCCUS-LACTIS

Lactococcus lactis subsp. lactis ML3 contains high pools of proline or betaine when grown under conditions of high osmotic strength. These p ools are created by specific transport systems. A high-affinity uptake system for glycine betaine (betaine) with a K(i) of 1.5 muM is expres sed constitutively. The activity of this system is not stimulated by h igh osmolarities of the growth or assay medium but varies strongly wit h the medium pH. A low-affinity proline uptake system (K(m), > 5 mM) i s expressed at high levels only in chemically defined medium (CDM) wit h high osmolarity. This transport system is also stimulated by high os molarity. The expression of this proline uptake system is repressed in rich broth with low or high osmolarity, and in CDM with low osmolarit y. The accumulated proline can be exchanged for betaine. Proline uptak e is also effectively inhibited by betaine (K(i) of between 50 and 100 muM). The proline transport system therefore probably also transports betaine. The inhibition of proline transport by betaine results in lo w proline pools in cells grown in high-osmotic-strength, betaine-conta ining CDM. The energy and pH dependency and the influence of ionophore s on the activity of both transport systems suggest that these systems are not proton motive force driven. At low osmolarities, proline upta ke is low but significant. This low proline uptake is also inhibited b y betaine, although to a lesser extent than in cells grown in high-osm otic-strength CDM. These data indicate that proline uptake in L. lacti s is enzyme mediated and is not dependent on passive diffusion, as was previously believed.