Deleting up to 15 reductase genes lets E. coli retain di-nitro compounds and nitro-aldehydes

The abstract reports retention by class of compound. Di-nitro compounds and nitro-aldehydes held in the knockout strains, where wild-type E. coli reduces them ^[5]. The screen covered more than 20 nitroaromatic compounds, each supplied from outside the cell to wild-type and engineered strains ^[4]. The abstract does not say which compounds held, by how much, or over what incubation time.

Instability of nitroarenes in microbial cells is the barrier the work targets ^[1]. E. coli strips nitro groups fast through intracellular reduction ^[11]. The enzymes responsible, nitroreductases, are flavin-dependent reductases that also handle quinone reduction, redox mediation and oxidative-stress defence ^[12]. The activity is spread across many genes, so deleting one leaves it largely intact. That redundancy is the reason the team went to as many as 15 knockouts of known and candidate genes ^[3].

With the reductions removed, the strains can be used to make nitro compounds instead of destroying them. The team ran three transformations: converting an amine precursor to a nitro compound, synthesising nitrobenzaldehydes, and placing nitrophenylalanines into a translated protein from supplied nitrobenzaldehydes ^[6].

The industrial case for a biological route is in the comparison with current practice. Nitroarenes are used across pharmaceuticals, agrochemicals, dyes and energetic materials ^[7]. Nitration still runs on nitric acid and nitric-sulfuric mixtures, which demand careful heat management, generate environmental hazards, and constrain regioselectivity and functional group tolerance ^[8]. An enzyme route works in water with enzyme-defined selectivity, and in the best case uses molecular oxygen as co-reactant and leaves water as the only byproduct ^[9].

That best case depends on having nitrating enzymes to deploy, and there are not many. Nature has yielded roughly 200 nitro-containing natural products ^[10], and the known enzymes fall into three families ^[15]. A host that holds a nitro group long enough to measure is a prerequisite for screening new ones.

Earlier work had shown that deleting multiple NTR genes keeps certain pro-drugs and photocaged amino acids intact ^[13], and the link between NTR deletions and stability was known even if its extent was not ^[2]. What this adds is the systematic version. The field had not had a measure of nitroarene stability across a panel of chemistries as a function of how many genes come out and how long the cells incubate ^[14].