Escherichia coli mutants impaired in maltodextrin transport

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Abstract
Wild-type E. coli K-12 grew equally well on maltose and on maltodextrins containing up to 7 glucose residues. Three classes of mutant unable to grow on maltodextrins but still able to grow on maltose were investigated in detail. The first class, already known, was composed of phage .lambda.-resistant mutants, which lack the outer membrane protein coded by gene lamB. These mutants grow on maltose and maltotriose but not at all on maltotetraose and longer maltodextrins which cannot cross the outer membrane. A 2nd class of mutants were affected in malE, the structural gene of the periplasmic maltose binding protein. The maltose binding proteins isolated from the new mutants were altered in their substrate binding properties, but not in a way that could account for the mutant phenotypes. The results of growth experiments and transport studies suggest that these malE mutants are impaired in their ability to transport maltodextrins across the outer membrane. This implies that the maltose binding protein (in wild-type strains) cooperates with the .lambda. receptor in permeation through the outer membrane. The last class of mutants described were affected in malG, or perhaps in an as yet undetected gene close to malG. They were defective in the transfer of maltodextrins from the periplasmic space to the cytoplasm but only slightly affected in the transport of maltose.