PP_1703

Summary: The sulfite-reductase call misassigns the fused nitrate-reductase protein.

Reason: The full architecture contains a molybdenum nitrate-reductase catalytic region linked to a diflavin reductase module. Native KT2442 disruption/complementation identifies the historical nitrate-reductase gene, whose encoded 724-residue segment is exactly the Q88M71 N-terminal region. The reductase module alone resembles electron-input modules of sulfite reductases, but does not establish sulfite reduction by this fusion. This conclusion does not rely merely on absence of a siroheme domain.

Summary: A flavodoxin-like domain supports FMN binding.

Reason: PROSITE PS50902 identifies the flavodoxin-like region; this independent signature supports the cofactor assignment despite conflicting ARBA reaction statements.

Summary: The TreeGrafter membrane location does not fit this fused pyridine-nucleotide-linked assimilatory nitrate reductase; the supported compartment is the cytoplasm.

Reason: The TreeGrafter source is the node PANTHER:PTN000177398 in PTHR43105, whose membrane-bound members are respiratory nitrate reductase and complex I subunits. The only membrane IBD in the family's PAINT data sits at the deeper node PTN008678681, seeded by NarG (P09152), NarZ (P19319), NuoG (P33602) and a Candida albicans seed from the Candida Genome Database (CGD:CAL0000186451). It is cited here as context for where the membrane claim originates, not as this row's source node. Two independent lines place Q88M71 in the cytoplasm instead. First, it is an assimilatory (Nas) enzyme, the class described as cytoplasmic in contrast with membrane-bound Nar and periplasmic Nap. Second, its own architecture fuses the Mo-bisMGD catalytic domain to flavodoxin and FAD-binding FR-type diaphorase domains that take electrons directly from cytoplasmic NAD(P)H, and the record annotates no transmembrane segment or signal peptide. The same correction was made for the sibling fused enzyme NasN (A4ZHT3), where cytoplasmic fractions were measured directly. No KT2440 localization experiment exists, and a transient peripheral contact is not excluded, but no membrane-resident activity is supported.

Proposed replacements: cytoplasm

Summary: Refine oxidoreductase activity to nitrate reductase [NAD(P)H] activity.

Reason: The catalytic identity bridge and fused NAD(P)/flavin electron-input architecture support the nitrate-reductase class. NADPH preference measured for mycobacterial NasN is not transferred to KT2440.

Proposed replacements: nitrate reductase [NAD(P)H] activity

Summary: The nitrate-reductase region binds a molybdopterin cofactor.

Reason: Independent Pfam molybdopterin-domain classification supports binding; this does not rely on the ARBA-only cofactor prose.

Summary: The fused pyridine-nucleotide input module supports NAD(P)H-linked nitrate reduction.

Reason: The molybdoenzyme catalytic region is joined to flavodoxin, FAD-binding and NAD(P)-binding domains. This architecture and comparative characterized NasN support the NAD(P)H class over a cytochrome-specific reaction. Neither an exclusive NADPH preference nor a monomeric assembly has been demonstrated for Q88M71.

Proposed replacements: nitrate reductase [NAD(P)H] activity

Summary: The N-terminal nitrate-reductase region contains a 4Fe-4S domain.

Reason: PROSITE PS51669 identifies the Mo/W bis-MGD-type 4Fe-4S domain.

Summary: The catalytic nitrate-reduction step directly participates in nitrate assimilation.

Reason: Retain the inherited proposal: the protein executes nitrate reduction, rather than only being necessary for growth. The historical KT2442 gene disruption and rescue map to the exact Q88M71 catalytic segment, while KT2440 studies provide locus/pathway context. The historical 724-residue clone is not a biochemical assay of the current 1341-residue fusion. This process is not an ancestor or descendant of another process proposed for this gene.