Structure and function of aerotolerant, multiple-turnover THI4 thiazole synthases

B Vitamins: An Update on Their Importance for Plant Homeostasis

B vitamins are a source of coenzymes for a vast array of enzyme reactions, particularly those of metabolism. As metabolism is the basis of decisions that drive maintenance, growth, and development, B vitamin-derived coenzymes are key components that facilitate these processes. For over a century, we have known about these essential compounds and have elucidated their pathways of biosynthesis, repair, salvage, and degradation in numerous organisms. Only now are we beginning to understand their importance for regulatory processes, which are becoming an important topic in plants. Here, I highlight and discuss emerging evidence on how B vitamins are integrated into vital processes, from energy generation and nutrition to gene expression, and thereby contribute to the coordination of growth and developmental programs, particularly those that concern maintenance of a stable state, which is the foundational tenet of plant homeostasis.

Directed Evolution of Aerotolerance in Sulfide-Dependent Thiazole Synthases

Sulfide-dependent THI4 thiazole synthases could potentially be used to replace plant cysteine-dependent suicide THI4s, whose high protein turnover rates make thiamin synthesis exceptionally energy-expensive. However, sulfide-dependent THI4s are anaerobic or microoxic enzymes and hence unadapted to the aerobic conditions in plants; they are also slow enzymes (kcat < 1 h-1). To improve aerotolerance and activity, we applied continuous directed evolution under aerobic conditions in the yeast OrthoRep system to two sulfide-dependent bacterial THI4s. Seven beneficial single mutations were identified, of which five lie in the active-site cleft predicted by structural modeling and two recapitulate features of naturally aerotolerant THI4s. That single mutations gave substantial improvements suggests that further advance under selection will be possible by stacking mutations. This proof-of-concept study established that the performance of sulfide-dependent THI4s in aerobic conditions is evolvable and, more generally, that yeast OrthoRep provides a plant-like bridge to adapt nonplant enzymes to work better in plants.

On the evolution of coenzyme biosynthesis

Covering: up to 2022The report provides a broad approach to deciphering the evolution of coenzyme biosynthetic pathways. Here, these various pathways are analyzed with respect to the coenzymes required for this purpose. Coenzymes whose biosynthesis relies on a large number of coenzyme-mediated reactions probably appeared on the scene at a later stage of biological evolution, whereas the biosyntheses of pyridoxal phosphate (PLP) and nicotinamide (NAD⁺) require little additional coenzymatic support and are therefore most likely very ancient biosynthetic pathways.

Using continuous directed evolution to improve enzymes for plant applications

Continuous directed evolution of enzymes and other proteins in microbial hosts is capable of outperforming classical directed evolution by executing hypermutation and selection concurrently in vivo, at scale, with minimal manual input. Provided that a target enzyme's activity can be coupled to growth of the host cells, the activity can be improved simply by selecting for growth. Like all directed evolution, the continuous version requires no prior mechanistic knowledge of the target. Continuous directed evolution is thus a powerful way to modify plant or non-plant enzymes for use in plant metabolic research and engineering. Here, we first describe the basic features of the yeast (Saccharomyces cerevisiae) OrthoRep system for continuous directed evolution and compare it briefly with other systems. We then give a step-by-step account of three ways in which OrthoRep can be deployed to evolve primary metabolic enzymes, using a THI4 thiazole synthase as an example and illustrating the mutational outcomes obtained. We close by outlining applications of OrthoRep that serve growing demands (i) to change the characteristics of plant enzymes destined for return to plants, and (ii) to adapt ("plantize") enzymes from prokaryotes-especially exotic prokaryotes-to function well in mild, plant-like conditions.