Figure 2. Graphical representation of assimilatory nitrate reduction in protists and α-proteobacteria.

A – The diagram shows the gene clusters of assimilatory, NAD(P)H-dependent nitrate reduction in bacteria and eukaryotes.

The various elements of Nas operon of Klebsiella [36] and the NiiA-NiaD operon in fungi [35] are colour coded as indicated in the quandrant on the top right. B – Possible molecular evolution of fungal NiaD nitrate reductase. Each domain is identified by a specific symbol - see the text for details. C – Representative distance tree of various proteins containing the bacterial FNR-like conserved domain. The tree was obtained with Neighbour Joining (maximal distance 0.9) using the DELTABLAST program [80] with methane monooxygenase subunit c of Methylocella silvestris (MMOc, Accession: YP_002361598) as query. This reductase subunit of methane monooxygenase contains a FNR-like domain similar to that of assimilatory nitrate reductases [43] lying in a sister group as indicated.