Nitrogen Acquisition Strategies Mediated by Insect Symbionts: A Review of Their Mechanisms, Methodologies, and Case Studies

Abstract

Nitrogen is usually a restrictive nutrient that affects the growth and development of insects, especially of those living in low nitrogen nutrient niches. In response to the low nitrogen stress, insects have gradually developed symbiont-based stress response strategies-biological nitrogen fixation and nitrogenous waste recycling-to optimize dietary nitrogen intake. Based on the above two patterns, atmospheric nitrogen or nitrogenous waste (e.g., uric acid, urea) is converted into ammonia, which in turn is incorporated into the organism via the glutamine synthetase and glutamate synthase pathways. This review summarized the reaction mechanisms, conventional research methods and the various applications of biological nitrogen fixation and nitrogenous waste recycling strategies. Further, we compared the bio-reaction characteristics and conditions of two strategies, then proposed a model for nitrogen provisioning based on different strategies.

Keywords: GS/GOGAT cycle; amino acid biosynthesis; biological nitrogen fixation; insect symbionts; nitrogenous waste recycling.

Figure 1

BNF reaction and its molecular mechanism. (A) Chemical reaction formula for BNF; (B) schematic diagram of dinitrogen reduction. Electrons are transferred from ferredoxin/flavodoxin via dinitrogenase reductase to dinitrogenase. At least 16 molecules of MgATP are consumed for each reduction of a dinitrogen.

Figure 2

Urea hydrolysis reaction and molecular mechanism diagram of urease activation. (A) Chemical reaction formula for urea hydrolysis; (B) urease activation model in vivo. The structural proteins encoded by ureA/B/C constitute an inactive apoprotein, denoted as (ureABC)₃, whose activation in vivo requires the participation of Ni²⁺, CO₂, GTP, and numerous urease accessory gene products. ureD, ureF, and ureG sequentially combine with (ureABC)₃ to form a ureABC–ureDFG complex. Alternatively, ureD, ureF, and ureG first form an ureDFG heterotrimer, and then combine with (ureABC)₃. After that, the active sites on the ureABC–ureDFG complex can bind to Ni²⁺ delivered by ureE accessory proteins. Carbon dioxide is used to form the carboxy-lysine metal ligands; GTP hydrolysis (occurring in ureG) powers the assembly of the metallocenters, drives the activation of urease, and releases all accessory proteins (involved in the next urease activation process, subsequently). This reaction finally forms three catalytic sites on the urease, each containing two Ni²⁺.