The MalT-dependent and malZ-encoded maltodextrin glucosidase of Escherichia coli can be converted into a dextrinyltransferase by a single mutation.

malZ is a member of the mal regulon. It is controlled by MatT, the transcriptional activator of the maltose system. MalZ has been purified and identified as an enzyme hydrolyzing maltotriose and longer maltodextrins to glucose and maltose. MalZ is dispensable for growth on maltose or maltodextrins. Mutants lacking amylomaltase (encoded by malQ), the major maltose utilizing enzyme, cannot grow on maltose, maltotriose, or maltotetraose, despite the fact that they contain an effective transport system and MalZ. From such a malQ mutant a pseudorevertant was isolated that was able to grow on maltose. The suppressor mutation was mapped in malZ. The mutant gene was cloned. It contained a Trp to Cys exchange at position 292 of the deduced protein sequence. Surprisingly, the purified mutant enzyme was still unable to hydrolyze maltose as was the wild type enzyme, while both were able to release glucose from maltodextrins. However, the mutant enzyme had gained the ability to transfer dextrinyl moieties to glucose, maltose, and other maltodextrins. Thus, it had gained an activity associated with amylomaltase. It was the MalZ292-associated transferase reaction that allowed the utilization of maltose. In addition, we discovered that mutant and wild type enzyme alike were highly active as gamma-cyclodextrinases.