(A) Putative NAD(P)-dependent dihydropyrimidine dehydrogenases containing a Pyr_redox_2 domain within the same polypeptide (blue) or in an adjacent ORF (red) are respectively colored. (B) Sequences from anaerobic bacteria in the order Clostridiales are colored green. Comparison of A and B show that many of the Clostridiales PydA homologs are not associated with a Pyr_redox_2 domain, suggesting that they do not use NAD(P)H as a reductant, and may instead use reduced Fdx as a reductant.