A recombinant enzyme, having a molecular weight of about 57,000-67,000 daltons on SDS-PAGE and a pI of about 4.1-5.1 on isoelectrophoresis, which converts maltose into trehalose and vice versa. Depending on the enzymatic conditions, the enzyme forms about 70 w/w % of trehalose when acts on maltose, while about 20 w/w % of maltose when acts on trehalose. The culture of a transformant, prepared by introducing into a host a recombinant DNA containing a DNA coding for the enzyme and a self-replicable vector, facilitates the industrial-scale production of trehalose.