Involved in pyrimidine base degradation. Catalyzes physiologically the reduction of uracil to 5,6-dihydrouracil (DHU) by using NADH as a specific cosubstrate. It also catalyzes the reverse reaction and the reduction of thymine to 5,6-dihydrothymine (DHT) (By similarity). 5,6-dihydrouracil + NAD(+) = H(+) + NADH + uracil 5,6-dihydrothymine + NAD(+) = H(+) + NADH + thymine Heterotetramer of 2 PreA and 2 PreT subunits. Belongs to the NADH dehydrogenase family.