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Bacteria exhibit a rich repertoire of RNA molecules that intricately regulate gene expression at multiple hierarchical levels, including small RNAs (sRNAs), riboswitches, and antisense RNAs. Notably, the majority of these regulatory RNAs lack or have limited protein-coding capacity but play pivotal roles in orchestrating gene expression by modulating transcription, post-transcription or translation processes. Leveraging and redesigning these regulatory RNA elements have emerged as pivotal strategies in the domains of metabolic engineering and synthetic biology. While previous investigations predominantly focused on delineating the roles of regulatory RNA in Gram-negative bacterial models such as Escherichia coli and Salmonella enterica, this review aims to summarize the mechanisms and functionalities of endogenous regulatory RNAs inherent to typical Gram-positive bacteria, notably Bacillus subtilis. Furthermore, we explore the engineering and practical applications of these regulatory RNA elements in the arena of synthetic biology, employing B. subtilis as a foundational chassis.

Gene regulation by trans-acting small RNAs (sRNAs) has considerable advantages over other gene regulation strategies. However, synthetic sRNAs mainly take natural sRNAs (MicC or SgrS) as backbones and comprise three functional elements folding into two or more stem-loop structures: an mRNA base pairing region, an Hfq-binding structure, and a rho-independent terminator. Due to limited numbers of natural sRNAs and complicated backbone structures, synthetic sRNAs suffer from low activity programmability and poor structural modularity. Moreover, natural sRNA backbone sequences may increase the possibility of unwanted recombination. Here, we present a bottom-up approach for creating structure defined single-stem loop small non-coding RNAs (ssl-sRNAs), which contain a standardized scaffold of a 7 bp-stem-4 nt-loop-polyU-tail and a 24 nt basing pairing region covering the first eight codons. Particularly, ssl-sRNA requires no independent Hfq-binding structure, as the polyU tail fulfills the roles of binding Hfq. A thermodynamic-based scoring model and a web server sslRNAD (http://www.kangzlab.cn/) were developed for automated design of ssl-sRNAs with well-defined structures and programmable activities. ssl-sRNAs displayed weak polar effects when regulating polycistronic mRNAs. The ssl-sRNA designed by sslRNAD showed regulatory activities in both Escherichia coli and Bacillus subtilis. A streamlined workflow was developed for the construction of customized ssl-sRNA and ssl-sRNA libraries. As examples, the E. coli cell morphology was easily modified and new target genes of ergothioneine biosynthesis were quickly identified with ssl-sRNAs. ssl-sRNA and its designer sslRNAD enable researchers to rapidly design sRNAs for knocking down target genes.

Bacillus subtilis as the Gram-positive model bacterium has been widely used in synthetic biology and biotechnology while the regulatory RNA tools for B. subtilis are still not fully explored. Here, a bottom-up approach is proposed for designing artificial trans-acting sRNAs. By engineering the intrinsic sRNA SR6, a minimized core scaffold structure consisting of an 8 bp stem, a 4 nt loop, and a 9 nt polyU tail was generated and proven to be sufficient for constructing sRNAs with strong repression activity (83%). Moreover, we demonstrate this artificial sRNA system functions well in an hfq-independent manner and also achieves strong repression efficiency in Escherichia coli (above 80%). A structure-based sRNA design principle was further developed for the automatic generation of custom sRNAs with this core scaffold but various sequences, which facilitates the manipulation and avoids structure disruption when fusing any base-pairing sequence. By applying these auto-designed sRNAs, we rapidly modified the cell morphology and biofilm formation, and regulated metabolic flux toward acetoin biosynthesis. This sRNA system with cross-species regulatory activities not only enriched the gene regulation toolkit in synthetic biology for B. subtilis and E. coli but also enhanced our understanding of trans-acting sRNAs.