The regulation of glycine betaine accumulation by was investigated. The accumulation of glycine betaine was regulated by the osmotic pressure of the medium and the low affinity transport system played the major role in this regulation. Mutants were isolated that lack the low affinity, osmotically activated glycine betaine/proline transport system. Such mutants accumulated glycine betaine via the high affinity system but the glycine betaine pool was smaller and responded poorly to osmotic pressure changes. The regulation of glycine betaine transport has revealed that at the steady state net influx is reduced and that this is achieved by inhibition of both the low affinity and the high affinity transport systems. Cells pre-loaded with glycine betaine exhibited a reduced for both systems: the low affinity system was reduced in activity fivefold and the high affinity system was reduced 10-fold and became virtually undetectable. Although glycine betaine transport at the steady state is reduced, retention of the compatible solute is an active process since addition of an uncoupler provokes rapid release of the accumulated material. These data suggest that feedback regulation of the activity of the uptake systems is a major mechanism for controlling the level of compatible solute accumulation.


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