L-tryptophan is an essential amino acid that is widely used in food, medicine and feed sectors. L-tryptophan can be produced through fermentation, and the main producing strains are engineered Escherichia coli and Corynebacterium glutamicum, which are constructed by rational design methods based on metabolic engineering and synthetic biology. However, due to the long metabolic pathway, complex and unclear regulatory mechanism for L-tryptophan production in microbial cells, the production efficiency and robustness of L-tryptophan producing strains are still low. In this connection, irrational design methods such as laboratory adaptive evolution, are often applied to improve the performance of L-tryptophan producing strains. This review summarizes the recent progress on biosynthesis metabolism of L-tryptophan and its regulation, the construction and optimization of L-tryptophan producing strains, and fermentative production of L-tryptophan, and prospects future development perspective. This review may facilitate research and development for fermentative production of L-tryptophan.

The acid signal transduction system can sense the acidic environment and translate it into signals to regulate various acid tolerance mechanisms within bacteria, helping them to cope with the stress of the acidic environment and survive the acidic environments. This review describes several major acid signal transduction systems that play important roles in acid-tolerant bacteria:EvgS/EvgA, PhoQ/PhoP, ArsS/ArsR, and CadC. The structural components of these systems and their regulation of acid-tolerant systems were used to analyze how acid-tolerant bacteria transduce signal in an acid environment to activate the corresponding acid-tolerance mechanisms and cope with the acid stress. An in-depth understanding of the regulatory mechanisms of acid-tolerant systems can help the mining, optimal design and construction of multiple acid-tolerant parts to improve the growth and metabolism of target strains in acidic environments. It helps to better utilize engineered microorganisms with super acid-resistance for industrial production of valuable metabolites, bioremediation of pollution in acidic environments. Moreover, it also helps to provide novel targets for inhibiting the growth of acid-tolerant pathogenic bacteria.

Erythritol is a novel 4-carbon sugar alcohol produced by microbes in the presence of hyper-osmotic stress. It has excellent potential to serve as an alternative sugar for people with diabetes and also a platform compound for synthesizing various C4 compounds, such as 1,3-butadiene, 1,4-butanediol, 2,5-dihydrofuran and so on. Compared with other polyols, the fermentative production of erythritol is more challenging. Yarrowia lipolytica is the preferred chassis of erythritol biosynthesis for its high-titer and high-productivity. At present, there are still some bottlenecks in the production of erythritol by Y. lipolytica, such as weak metabolic activity, abundant by-products, and low industrial attributes. Progress has been made in tailoring high version strains according to industrial needs. For example, the highest titer of erythritol produced by the metabolically engineered Y. lipolytica reached 196 g/L and 150 g/L, respectively, by using glucose or glycerol as the carbon sources. However, further improving its production performance becomes challenging. This review summarizes the research progress in the synthesis of erythritol by Y. lipolytica from the perspectives of erythritol producing strains, metabolic pathways, modular modifications, and auxiliary strategies to enhance the industrial properties of the engineered strain. Key nodes in the metabolic pathway and their combination strategies are discussed to guide the research on promoting the production of erythritol by Y. lipolytica.

Spider silk is a natural fiber known as "biosteel" with the strongest composite performance, such as high tensile strength and toughness. It is also equipped with excellent biocompatibility and shape memory ability, thus shows great potential in many fields such as biomedicine and tissue engineering. Spider silk is composed of macromolecular spidroin with rich structural diversity. The characteristics of the primary structure of natural spidroin, such as the high repeatability of amino acids in the core repetitive region, the high content of specific amino acids, the large molecular weight, and the high GC content of the spidroin gene, have brought great difficulties in heterologous expression. This review discusses focuses on the relationship between the featured motifs of the microcrystalline region in the repetitive unit of spidroin and its structure, as well as the spinning performance and the heterologous expression. The optimization design for the sequence of spidroin combined with heterologous expression strategy has greatly promoted the development of the biosynthesis of spider silk proteins. This review may facilitate the rational design and efficient synthesis of recombinant spidroin.