Nitrate and nitrite transport in bacteria

The topological arrangements of nitrate and nitrite reductases in bacteria necessitate the synthesis of transporter proteins that carry the Nitrogen o xyanions across the cytoplasmic membrane. For assimilation of nitrate land nitrite) there are two types of uptake system known: ABC transporters that are driven by ATP hydrolysis, and secondary transporters reliant on a proto n motive force. Proteins homologous to the latter type of transporter are a lso involved in nitrate and nitrite transport in dissimilatory processes su ch as denitrification. These proteins belong to the NarK family, which is a branch of the Major Facilitator Superfamily. The mechanism and substrate s pecificity of transport via these proteins is unknown, but is discussed in the light of sequence analysis of members of the NarK family. A hypothesis for nitrate and nitrite transport is proposed based on the finding that the re are two distinct types of NarK.