The colicin I receptor of Escherichia coli K‐12 has a role in enterochelin‐mediated iron transport

Iron can enter the E coZi K-l 2 cell in four ways. There exist three specific, high affinity systems which transport iron with the aid of either enterochelin [l] , citrate [2] or ferrichrome [3]. The fourth system is a low affinity uptake system for which no such complexing agent has been described [2]. Our knowledge of the membrane components required for high affinity iron transport and for mobilization of iron from the complexes is still poor, although the isolation of specific mutants in these uptake systems should assist in defining the required components. For example, it was shown that ton A mutants are missing a protein [4] and are defective in ferrichrome-mediated iron transport [3]. Ferrichrome protects sensitive cells against phages T5, Tl and $80 and colicin M, all of which bind to the ton A-protein [3,5,6]. Thus the ton A-protein is probably the initial binding receptor for ferrichrome. Other mutants have been described for the ferrichrome [3,4,5], citrate [2] and enterochelin (see [l] and [7] for summary) mediated transport systems, although in many cases the defects involved have not been fully characterized. Recently the isolation of feu mutants (defective in ferric enterochelin uptake) was described [8]. These mutants, which had essentially normal citrateand ferrichrome-mediated iron uptake systems, mapped separately from previously described iron transport mutants (at about 65 min on the E coli genetic map). They fell into two groups with respect to colicin resistance, those insensitive to colicins I and V (called here feu A) and those insensitive to colicins B and I (called here