Involved in galactitol and D-altritol catabolism. Catalyzes the phosphorylation of D-tagatose to D-tagatose 6-phosphate. ATP + keto-D-tagatose = ADP + H(+) + keto-D-tagatose 6-phosphate kcat is 3.0 sec(-1). Carbohydrate metabolism. Up-regulated by growth on D-altritol or galactitol. Deletion of the gene does not result in growth defects for either D-galactitol or D-altritol. Belongs to the carbohydrate kinase PfkB family.