Molecular cloning of a maltose transport gene from Bacillus stearothermophilus and its expression in Escherichia coli K-12.

Genes responsible for maltose utilization from Bacillus stearothermophilus ATCC7953 were cloned in the plasmid vector pBR325 and functionally expressed in Escherichia coli. The 4.2 kb Bacillus DNA insert in clone pAM1750 suppressed the growth defects on maltose caused by mutations in E. coli maltose transport genes (malE, malK or complete malB deletion) but not mutations in genes affecting intracellular maltose metabolism (malA region). Transport studies in E. coli and B. stearothermophilus suggested that pAM1750 codes for a high affinity transport system, probably one of two maltose uptake systems found in B. stearothermophilus ATCC7953. Nucleotide sequence analysis of a 3.6 kb fragment of pAM1750 revealed three open reading frames (ORFs). One of the ORFs, malA, encoded a putative hydrophobic protein with 12 potential transmembrane segments. MalA showed amino acid sequence similarity to proteins in the superfamily containing LacY lactose permease and also some similarity to MalG protein, a member of a binding protein-dependent transport system in E. coli. The products of two other ORFs were not hydrophobic, did not show similarity to other known sequences and were found not to be essential for maltose utilization in transport-defective E. coli mutants. Hence MalA protein was the only protein necessary for maltose transport, but despite giving a detectable but low level of transport function in E. coli, the protein was very poorly expressed and could not be identified.