Molecular characterization of the MalT-dependent periplasmic alpha-amylase of Escherichia coli encoded by malS.

malS, the gene encoding the periplasmic alpha-amylase, is under the regulatory control of the MalT protein, the gene activator of the Escherichia coli maltose system. We sequenced the DNA region encoding malS and its control elements. malS consists of an open reading frame of 2,028 base pairs encoding a protein of 676 amino acids with a deduced molecular weight of 75,664 including a typical amino-terminal signal sequence of 17 amino acids. The deduced amino acid sequence of malS was compared with that of other proteins. We found homologies to alpha-amylases and a cyclodextrin glucanotransferase but not to beta-amylases. In addition, MalS showed significant homology to another maltodextrin-utilizing and MalT-dependent enzyme of E. coli, maltodextrin glucosidase (MalZ) but not to amylomaltase (MalQ), the major maltodextrin-degrading enzyme. Conserved regions that have been proposed to constitute enzymatically active sites in alpha-amylases are present in MalS. Two of these sequences can also be found in the amino terminus of the lambda-receptor, a maltodextrin-specific channel in the outer membrane. The nucleotide sequence of the control region of malS revealed MalT binding sites correctly spaced for the start of the malS transcript. At the position 219 base pairs upstream of malS, we found a divergently transcribed gene, bax, which has been recognized previously. Downstream of malS, after a 513-base pair intergenic region, lies the convergently transcribed gene avtA, which codes for the alanine/valine transaminase.