Reprogramming complex cellular metabolism requires simultaneous regulation of multigene expression. Ex-situ cloning-based methods are commonly used, but the target gene number and combinatorial library size are severely limited by cloning and transformation efficiencies. In-situ methods such as multiplex automated genome engineering (MAGE) depends on high-efficiency transformation and incorporation of heterologous DNA donors, which are limited to few microorganisms. Here, we describe a Base Editor-Targeted and Template-free Expression Regulation (BETTER) method for simultaneously diversifying multigene expression. BETTER repurposes CRISPR-guided base editors and in-situ generates large numbers of genetic combinations of diverse ribosome binding sites, 5’ untranslated regions, or promoters, without library construction, transformation, and incorporation of DNA donors. We apply BETTER to simultaneously regulate expression of up to ten genes in industrial and model microorganisms Corynebacterium glutamicum and Bacillus subtilis. Variants with improved xylose catabolism, glycerol catabolism, or lycopene biosynthesis are respectively obtained. This technology will be useful for large-scale fine-tuning of multigene expression in both genetically tractable and intractable microorganisms.

CRISPR/Cas9‐guided cytidine deaminase enables C:G to T:A base editing in bacterial genome without introduction of lethal double‐stranded DNA break, supplement of foreign DNA template, or dependence on inefficient homologous recombination. However, limited by genome‐targeting scope, editing window, and base transition capability, the application of base editing in metabolic engineering has not been explored. Herein, four Cas9 variants accepting different protospacer adjacent motif (PAM) sequences were used to increase the genome‐targeting scope of bacterial base editing. After a comprehensive evaluation, we demonstrated that PAM requirement of bacterial base editing can be relaxed from NGG to NG using the Cas9 variants, providing 3.9‐fold more target loci for gene inactivation in Corynebacterium glutamicum. Truncated or extended guide RNAs were employed to expand the canonical 5‐bp editing window to 7‐bp. Bacterial adenine base editing was also achieved with Cas9 fused to adenosine deaminase. With these updates, base editing can serve as an enabling tool for fast metabolic engineering. To demonstrate its potential, base editing was used to deregulate feedback inhibition of aspartokinase via amino acid substitution for lysine overproduction. Finally, a user‐friendly online tool named gBIG was provided for designing guide RNAs for base editing‐mediated inactivation of given genes in any given sequenced genome (http://www.ibiodesign.net/gBIG).

The unusual high activity of BmGAD at pH 6 makes it an attractive GABA-producing candidate in industrial application.