Streptomycetes: Attractive Hosts for Recombinant Protein Production

Abstract

Enzymes are increasingly applied as biocatalysts for fulfilling industrial needs in a variety of applications and there is a bursting of interest for novel therapeutic proteins. Consequently, developing appropriate expression platforms for efficiently producing such recombinant proteins represents a crucial challenge. It is nowadays widely accepted that an ideal 'universal microbial host' for heterologous protein expression does not exist. Indeed, the first-choice microbes, as Escherichia coli or yeasts, possess known intrinsic limitations that inevitably restrict their applications. In this scenario, bacteria belonging to the Streptomyces genus need to be considered with more attention as promising, alternative, and versatile platforms for recombinant protein production. This is due to their peculiar features, first-of-all their natural attitude to secrete proteins in the extracellular milieu. Additionally, streptomycetes are considered robust and scalable industrial strains and a wide range of tools for their genetic manipulation is nowadays available. This mini-review includes an overview of recombinant protein production in streptomycetes, covering nearly 100 cases of heterologous proteins expressed in these Gram-positives from the 1980s to December 2019. We investigated homologous sources, heterologous hosts, and molecular tools (promoters/vectors/signal peptides) used for the expression of these recombinant proteins. We reported on their final cellular localization and yield. Thus, this analysis might represent a useful source of information, showing pros and cons of using streptomycetes as platform for recombinant protein production and paving the way for their more extensive use in future as alternative heterologous hosts.

Streptomyces as Microbial Chassis for Heterologous Protein Expression

“…The growth of several Streptomyces species is robust and scalable, as they have been intensively used for the industrial production of SMs (Berini et al, 2020). They are also favorable hosts for heterologous production of recombinant proteins.…”

“…They are also favorable hosts for heterologous production of recombinant proteins. This is mainly due to their protein secretion systems in the extracellular milieu (Liu et al, 2013;Hamed et al, 2018;Berini et al, 2020). Secretion is beneficial in terms of protein folding, because the extracellular medium is an ordinarily oxidizing environment that promotes disulfide bond formation for correct folding, while the cytoplasm is a reducing environment that impedes the disulfide bonds (Anne et al, 2014).…”

“…As the cell disruption is not required, the downstream protein purification from secreted proteins by Streptomyces is more comfortable without contamination by intracellular proteins (Sommer et al, 2009). In addition, Streptomyces exhibits low toxicity and does not contain lipopolysaccharides (LPSs), which may act as potent immunostimulatory endotoxins (Berini et al, 2020), allowing easy downstream purification processes. Moreover, S. lividans, the most frequently used Streptomyces host for recombinant protein production, is known to have low restriction enzyme and proteolytic activities, enabling efficient introduction of the recombinant DNA and high yield of the protein, respectively (Anne et al, 2014).…”

“…To survive under limited nutrient conditions and compete with various other microorganisms in soil, Streptomyces has a complex developmental cycle that sporulates after mycelial growth and produces diverse SMs (Van Wezel and Mcdowall, 2011;Barbuto Ferraiuolo et al, 2021). In addition, various enzymes are known to be produced and secreted to degrade complex nutrients, such as lignocellulose, and to perform various reactions on different substrates (Berini et al, 2020). SMs are synthesized by a series of reactions catalyzed by the biosynthetic enzymes encoded in the BGCs.…”

Advances in Biosynthesis of Natural Products from Marine Microorganisms

“…Selecting a suitable host and an appropriate vector is crucial for successful heterologous expression [67]. For the expression of BGCs from bacteria, such as Streptomyces and other actinomycetes, laboratory Streptomyces strains, such as S. coelicolor, S. lividans, and S. albus, are favorable because of compatible codon and promoter usage [68]. Recently, microalgae have been applied as heterologous hosts to produce proteins at high expression levels through sequence optimization [69] and using secretory signal peptides [70].…”