Nitrilase mediated conversion of Indole-3-acetonitrile to Indole-3-acetic acid

The Plant Growth-Promoting Rhizobacterium Variovorax boronicumulans CGMCC 4969 Regulates the Level of Indole-3-Acetic Acid Synthesized from Indole-3-Acetonitrile

Variovorax is a metabolically diverse genus of plant growth-promoting rhizobacteria (PGPR) that engages in mutually beneficial interactions between plants and microbes. Unlike most PGPR, Variovorax cannot synthesize the phytohormone indole-3-acetic acid (IAA) via tryptophan. However, we found that Variovorax boronicumulans strain CGMCC 4969 can produce IAA using indole-3-acetonitrile (IAN) as the precursor. Thus, in the present study, the IAA synthesis mechanism of V. boronicumulans CGMCC 4969 was investigated. V. boronicumulans CGMCC 4969 metabolized IAN to IAA through both a nitrilase-dependent pathway and a nitrile hydratase (NHase) and amidase-dependent pathway. Cobalt enhanced the metabolic flux via the NHase/amidase, by which IAN was rapidly converted to indole-3-acetamide (IAM) and in turn to IAA. IAN stimulated metabolic flux via the nitrilase, by which IAN was rapidly converted to IAA. Subsequently, the IAA was degraded. V. boronicumulans CGMCC 4969 can use IAN as the sole carbon and nitrogen source for growth. Genome sequencing confirmed the IAA synthesis pathways. Gene cloning and overexpression in Escherichia coli indicated that NitA has nitrilase activity and IamA has amidase activity to respectively transform IAN and IAM to IAA. Interestingly, NitA showed a close genetic relationship with the nitrilase of the phytopathogen Pseudomonas syringae Quantitative PCR analysis indicated that the NHase/amidase system is constitutively expressed, whereas the nitrilase is inducible. The present study helps our understanding of the versatile functions of Variovorax nitrile-converting enzymes that mediate IAA synthesis and the interactions between plants and these bacteria.IMPORTANCE We demonstrated that Variovorax boronicumulans CGMCC 4969 has two enzymatic systems-nitrilase and nitrile hydratase/amidase-that convert indole-3-acetonitrile (IAN) to the important plant hormone indole-3-acetic acid (IAA). The two IAA synthesis systems have very different regulatory mechanisms, affecting the IAA synthesis rate and duration. The nitrilase was induced by IAN, which was rapidly converted to IAA; subsequently, IAA was rapidly consumed for cell growth. The nitrile hydratase (NHase) and amidase system was constitutively expressed and slowly but continuously synthesized IAA. In addition to synthesizing IAA from IAN, CGMCC 4969 has a rapid IAA degradation system, which would be helpful for a host plant to eliminate redundant IAA. This study indicates that the plant growth-promoting rhizobacterium V. boronicumulans CGMCC 4969 has the potential to be used by host plants to regulate the IAA level.

Bioengineering of Nitrilases Towards Its Use as Green Catalyst: Applications and Perspectives

Nitrilases are commercial biocatalysts used for the synthesis of plastics, paints, fibers in the chemical industries, pharmaceutical drugs and herbicides for agricultural uses. Nitrilase hydrolyses the nitriles and dinitriles to their corresponding carboxylic acids and ammonia. They have a broad range of substrate specificities as well as enantio-, regio- and chemo-selective properties which make them useful for biotransformation of nitriles to important compounds because of which they are considered as 'Green Catalysts'. Nitriles are widespread in nature and synthesized as a consequence of anthropogenic and biological activities. These are also present in certain plant species and are known to cause environmental pollution. Biotransformation using native organisms as catalysts tends to be insufficient since the enzyme of interest has very low amount in the total cellular protein, rate of reaction is slow along with the instability of enzymes. Therefore, to overcome these limitations, bioengineering offers an alternative approach to alter the properties of enzymes to enhance the applicability and stability. The present review highlights the aspects of producing the recombinant microorganisms and overexpressing the enzyme of interest for the enhanced stability at high temperatures, immobilization techniques, extremes of pH, organic solvents and hydrolysing dintriles to chiral compounds which may enhance the possibilities for creating specific enzymes for biotransformation.

Ammonium acrylate biomanufacturing by an engineered Rhodococcus ruber with nitrilase overexpression and double-knockout of nitrile hydratase and amidase

Rhodococcus ruber TH was selected as a parent strain to engineer for biomanufacturing of ammonium acrylate; the characteristics of this strain included accelerated growth rate, high cell tolerance and natively overexpressed nitrile hydratase (NHase). Transcriptome analysis revealed that the transcription levels of the native NHase, amidase and nitrilase were extremely high, moderate and extremely low, respectively. Through NHase-amidase double-knockout and amidase single-knockout, the engineered strains R. ruber THdAdN and R. ruber THdA were obtained for overexpression of a heterologous nitrilase from R. rhodochrous tg1-A6 using a urea-induced Pa2 promoter. The nitrilase activity toward substrate acrylonitrile in the engineered THdAdN(Nit) reached 187.0 U/mL at 42 h, threefold of that R. rhodochrous tg1-A6 and 2.3-fold of that of THdA(Nit). The optimal catalysis temperature and pH of the nitrilases in different cells exhibited no significant difference. Using the cells as catalysts, biomanufacturing of ammonium acrylate was performed under room temperature. When catalyzed by the engineered THdAdN(Nit), the titer and productivity of ammonium acrylate dramatically increased to 741.0 g/L and 344.9 g/L/h, which are the highest results reported to date.