A new family of nitrate/nitrite transporters involved in denitrification

Denitrifying bacteria carry out nitrate and nitrite respiration inside and outside the cell, respectively. In Thermus thermophilus, nitrate and nitrite transport processes are carried out by major facilitator superfamily (MFS) transporters. The sequence of the nar operon of nitrate-only respiring strains of T. thermophilus includes two tandemly organized MFS transporter genes (narK and narT) of the NarK1 and NarK2 families. Both can function as nitrate/nitrite antiporters, but NarK has been proposed as more specific for nitrate whereas NarT more specific for nitrite. In some nitrate- and nitrite-respiring strains of the same species, a single MFS transporter (NarO) belonging to a different MFS subfamily appears. To analyze the role of this single MFS in the same genetic context, we transferred the two types of nar operon to the aerobic strain HB27, and further included in both of them the ability to respire nitrite. The new denitrifying strains HB27dn, with two MFS, and HB27dp, with a single one, were used to isolate mutants devoid of transporters. Through in trans complementation experiments, we demonstrate that the NarO single MFS works efficiently in the transport of both nitrate and nitrite.