The First Step of Neurospora crassa Molybdenum Cofactor Biosynthesis: Regulatory Aspects under N-Derepressing and Nitrate-Inducing Conditions.

Tobias Kruse,Robert Hänsch,Simon Wajmann,Sabine Buchmeier,Thomas W. Hercher,Ralf R. Mendel

doi:10.3390/microorganisms8040534

Tobias Kruse, Robert Hänsch

Open Access

https://doi.org/10.3390/microorganisms8040534

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Abstract

Molybdenum cofactor (Moco) is the active site prosthetic group found in all Moco dependent enzymes, except for nitrogenase. Mo-enzymes are crucial for viability throughout all kingdoms of life as they catalyze a diverse set of two electron transfer reactions. The highly conserved Moco biosynthesis pathway consists of four different steps in which guanosine triphosphate is converted into cyclic pyranopterin monophosphate, molybdopterin (MPT), and subsequently adenylated MPT and Moco. Although the enzymes and mechanisms involved in these steps are well characterized, the regulation of eukaryotic Moco biosynthesis is not. Within this work, we described the regulation of Moco biosynthesis in the filamentous fungus Neurospora crassa, which revealed the first step of the multi-step pathway to be under transcriptional control. We found, that upon the induction of high cellular Moco demand a single transcript variant of the nit-7 gene is increasingly formed pointing towards, that essentially the encoded enzyme NIT7-A is the key player for Moco biosynthesis activity in Neurospora.

Highlights

In order to gain biological activity, molybdenum (Mo) needs to be coordinated by molybdopterin (MPT), resulting in the formation of the molybdenum cofactor (Moco)
Nitrate reductase expression is connected to Moco biosynthesis regulation—As nitrate reductase activity depends on active site bound Moco, biosynthesis of Moco is a requirement for nitrate assimilation
Whereas the regulation of nitrate reductase (NR) is well characterized in the fungi Aspergillus nidulans (A. nidulans) and N. crassa [22], regulation of Moco biosynthesis is not, and notably, this holds true for any other eukaryote

Summary

Introduction

In order to gain biological activity, molybdenum (Mo) needs to be coordinated by molybdopterin (MPT), resulting in the formation of the molybdenum cofactor (Moco). In the second step of Moco biosynthesis, the dithiolene motif is introduced into cPMP, forming the Mo chelating scaffold molybdopterin (MPT) [10,11,12]. This reaction is performed by the heterotetrameric MPT synthase complex, which consists of two sulfur carrier subunits (N. crassa NIT-8) and two catalytic subunits (N. crassa NIT-12) [1,3,7]. The NIT-9G domain adenylates MPT and the NIT-9E domain accepts the synthesized MPT-AMP as substrate for the subsequent molybdate insertion reaction, resulting in the formation of physiologically active Moco [1,3,7] (Figure 1)

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