Promoter regulation and genetic engineering strategies for enhanced cellulase expression in Trichoderma reesei

Background: Trichoderma reesei has extraordinary potential for high-level protein production at large scales, and it need to be further explored through genetic engineering tools to obtain a thorough understanding of its cellular physiology. Understanding the genetic factors involved in the intrinsic regulatory network is crucial; without this information, there would be restrictions in expressing genes of interest. Past and present studies are concentrated on the application and expansion of novel expression systems using synthetic biology concepts. These approaches involve either using previously established promoters that are strong or genetically engineered promoters. Genomic and transcriptomic methods have also been employed to isolate strong promoters and expression systems such as light-inducible expression systems, copper-inducible expression systems, L-methionine inducible promoters, and Tet-On expression system etc. Aims of review: In this review, we will highlight various research endeavors related to tunable and constitutive promoters; the role of different promoters in homologous and heterologous protein expression; the identification of innovative promoters, and strategies that may be beneficial for future research aimed at improving and enhancing protein expression in T. reesei. Key scientific concepts of the review: The characterization of new promoters and implementation of novel expression systems that will result in a significant extension of the molecular toolbox that is accessible for the genetic engineering of innovative strains of T. reesei. Genetically engineered strong inducible promoters such as Pcbh1 through replacement of transcriptional repressors (cre1, ace1) with transcriptional activators (xyr1, ace2, ace3, hap2/3/5) and synthetic expression systems can result in elevated production of endoglucanases (EGLs), beta-glucosidases (BGLs), and cellobiohydrolases (CBHs). Strong constitutive promoters such as Pcdna1 can be converted into genetically engineered synthetic hybrid promoters by integrating the activation region of strong inducible promoters, which can allow the induction and expression of cellulases even on repressing media. More efforts are necessary to identify innovative promoters and novel expression strategies for the enhanced expression of desirable proteins at industrial scales.