Methionine transport in Escherichia coli: physiological and genetic evidence for two uptake systems.

At least two transport systems serve for the entry of l-methionine into Escherichia coli. One of these systems has a high affinity for methionine (K(T) of about 10(-7) M) and is inhibited by methionyl peptides. The other system has a lower affinity (K(T) of 40 muM). Mutants defective in metD lack the high-affinity system and are unable to utilize d-methionine or N-acetylmethionine as methionine sources. A mutant was isolated which not only lacks the high-affinity system, but which also exhibits decreased activity of the low-affinity system. The transport parameters of the activity remaining in this strain were measured. The energy source for transport appears to be the same as that in the parental strain. The loss of the transport activities was not accompanied by major changes in the methionine pool size or in the rate of methionine excretion or synthesis. The analysis of revertants and transduction crosses supports the conclusion that the strain described is a double mutant in both metD (which codes for the high-affinity system) and metP (the low-affinity system). The metD locus was found to lie between tonA (38% co-transduction) and proA (9% co-transduction).