Treiber-Kleinke, C. | Search bioRxiv

bioRxiv2023

Escherichia coli adapts metabolically to 6- and 7-fluoroindole, enabling proteome-wide fluorotryptophan substitution

Nature has scarcely evolved a biochemistry around fluorine. However, modern science proved fluorinated organic molecules to be suitable building blocks for biopolymers, from peptides and proteins up to entire organisms. Here, we conducted adaptive laboratory evolution (ALE) experiments to introduce fluorine into living microorganisms. By cultivating Escherichia coli with fluorinated indole analogues, we successfully evolved microbial cells capable of utilizing either 6-fluoroindole or 7-fluoroindole for growth. Our improved ALE protocols enabled us to overcome previous challenges and achieve consistent and complete adaptation of microbial populations to these unnatural molecules. In the ALE experiments, we supplied fluoroindoles to auxotrophic E. coli bacteria, exerting strong selective pressure that led to microbial adaptation and growth on monofluorinated indoles. Within the cells, these indoles were converted into corresponding amino acids (6- and 7-fluorotryptophan) and incorporated into the proteome at tryptophan sites. This study is a first step and establishes a strong foundation for further exploration of the mechanisms underlying fluorine-based life and how a formerly stressor (fluorinated indole) becomes a vital nutrient. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=64 SRC="FIGDIR/small/559291v1_ufig1.gif" ALT="Figure 1"> View larger version (17K): org.highwire.dtl.DTLVardef@1a3b17org.highwire.dtl.DTLVardef@1d716aorg.highwire.dtl.DTLVardef@10126d5org.highwire.dtl.DTLVardef@9c8a43_HPS_FORMAT_FIGEXP M_FIG C_FIG

synthetic biology↗

bioRxiv2021

Evolving a mitigation of the stress response pathway to change the basic chemistry of life

Billions of years of evolution have produced only slight variations in the standard genetic code, and the number and identity of proteinogenic amino acids have remained mostly consistent throughout all three domains of life. These observations suggest a certain rigidity of the genetic code and prompt musings as to the origin and evolution of the code. Here we conducted an adaptive laboratory evolution (ALE) to push the limits of the code restriction, by evolving Escherichia coli to fully replace tryptophan, thought to be the latest addition to the genetic code, with the analog L-{beta}-(thieno[3,2-b]pyrrolyl)alanine ([3,2]Tpa). We identified an overshooting of the stress response system to be the main inhibiting factor for limiting ancestral growth upon exposure to {beta}-(thieno[3,2-b]pyrrole ([3,2]Tp), a metabolic precursor of [3,2]Tpa, and Trp limitation. During the ALE, E. coli was able to "calm down" its stress response machinery, thereby restoring growth. In particular, the inactivation of RpoS itself, the master regulon of the general stress response, was a key event during the adaptation. Knocking out the rpoS gene in the ancestral background independent of other changes conferred growth on [3,2]Tp. Our results add additional evidence that frozen regulatory constraints rather than a rigid protein translation apparatus are Lifes gatekeepers of the canonical amino acid repertoire. This information will not only enable us to design enhanced synthetic amino acid incorporation systems but may also shed light on a general biological mechanism trapping organismal configurations in a status quo. SIGNIFICANCE STATEMENTThe (apparent) rigidity of the genetic code, as well as its universality, have long since ushered explorations into expanding the code with synthetic, new-to-nature building blocks and testing its boundaries. While nowadays even proteome-wide incorporation of synthetic amino acids has been reported on several occasions1-3, little is known about the underlying mechanisms. We here report ALE with auxotrophic E. coli that yielded successful proteome-wide replacement of Trp by its synthetic analog [3,2]Tpa accompanied with the selection for loss of RpoS4 function. Such laboratory domestication of bacteria by the acquisition of rpoS mitigation mutations is beneficial not only to overcome the stress of nutrient (Trp) starvation but also to evolve the paths to use environmental xenobiotics (e.g. [3,2]Tp) as essential nutrients for growth. We pose that regulatory constraints rather than a rigid and conserved protein translation apparatus are Lifes gatekeepers of the canonical amino acid repertoire (at least where close structural analogs are concerned). Our findings contribute a step towards understanding possible environmental causes of genetic changes and their relationship to evolution. Our evolved strain affords a platform for homogenous protein labeling with [3,2]Tpa as well as for the production of biomolecules5, which are challenging to synthesize chemically. Top-down synthetic biology will also benefit greatly from breaking through the boundaries of the frozen bacterial genetic code, as this will enable us to begin creating synthetic cells capable to utilize an expanded range of substrates essential for life.

synthetic biology↗