Biology subjects

Horton, J. S.

Publications and source records attributed to Horton, J. S..

3 recordsLinked to original sources

bioRxiv2024

GnT motifs: short nucleotide tracts of greater than or equal to 8bp that can increase T-G:C mutation rates >1000-fold in bacteria

Nucleotides across a genome do not mutate at equal frequencies. Instead, specific nucleotide positions can exhibit much higher mutation rates than the genomic average due to their immediate nucleotide neighbours. These mutational hotspots can play a prominent role in adaptive evolution, yet we lack knowledge of which short nucleotide tracts drive hotspots. In this work, we employ a combinatorial approach of experimental evolution with Pseudomonas fluorescens and bioinformatic analysis of various Salmonella species to characterise a short nucleotide motif ([≥]8bp) that drives T:A[->]G:C mutation rates >1000-fold higher than the average T[->]G rate in bacteria. First, we experimentally show that homopolymeric tracts ([≥]3) of G with a 3 T frequently mutate so that the 3 T is replaced with a G, resulting in an extension of the guanine tract, i.e., GGGGT [->] GGGGG. We then demonstrate that the potency of this T:A[->]G:C hotspot is dependent on the nucleotides immediately flanking the GnT motif. We find that the dinucleotide pair immediately 5 to a G4 tract and the nucleotide immediately 3 to the T strongly affect the T:A[->]G:C mutation rate, which ranges from [~]5-fold higher than the typical rate to >1000-fold higher depending on the flanking elements. Therefore the T:A[->]G:C hotspot motif is a product of several modular nucleotide components (1-4bp in length) which each exert a significant effect on the mutation rate of the GnT motif. This work advances our ability to accurately identify the position and quantify the mutagenicity of hotspot motifs predicated on short tracts of nucleotides.

molecular biology↗

bioRxiv2023

Mutational hotspots lead to robust but suboptimal adaptive outcomes in certain environments

The observed mutational spectrum of adaptive outcomes can be constrained by many factors. For example, mutational biases can narrow the observed spectrum by increasing the rate of mutation at isolated sites in the genome. In contrast, complex environments can shift the observed spectrum by defining fitness consequences of mutational routes. We investigate the impact of different nutrient environments on the evolution of motility in the bacteria Pseudomonas fluorescens Pf0-2x (an engineered non-motile derivative of Pf0-1) in the presence and absence of a strong mutational hotspot. Previous work has shown that this mutational hotspot can be built and broken via six silent mutations, which provide rapid access to a mutation that rescues swimming motility and confers the strongest swimming phenotype in specific environments. Here, we evolved a hotspot and non-hotspot variant strain of Pf0-2x for motility under nutrient rich (LB) and nutrient limiting (M9) environmental conditions. We observed the hotspot strain consistently evolved faster across all environmental conditions and its mutational spectrum was robust to environmental differences. However, the non-hotspot strain had a distinct mutational spectrum that changed depending on the nutrient environment. Interestingly, while alternative adaptive mutations in nutrient-rich environments were equal to, or less effective than, the hotspot mutation, the majority of these mutations in nutrient-limited conditions produced superior swimmers. Our competition experiments mirrored these findings, underscoring the role of environment in defining both the mutational spectrum and the associated phenotype strength. This indicates that while mutational hotspots working in concert with natural selection can speed up access to robust adaptive mutations (which can provide a competitive advantage in evolving populations) they can limit exploration of the mutational landscape, restricting access to potentially stronger phenotypes in specific environments. DATA SUMMARYAll data is available at OSF: DOI 10.17605/OSF.IO/8BT2W, and in supplementary materials.

evolutionary biology↗

bioRxiv2021

Extreme parallel evolution of flagellar motility is facilitated by synonymous sequence

Mutational hotspots can determine evolutionary outcomes and make evolution repeatable. Hotspots are products of multiple evolutionary forces including mutation rate heterogeneity, but this variable is often hard to identify. In this work we reveal that a powerfully deterministic genetic hotspot can be built and broken by a handful of silent mutations. We observed this when studying homologous immotile variants of the bacteria Pseudomonas fluorescens, AR2 and Pf0-2x. AR2 resurrects motility through highly repeatable de novo mutation of the same nucleotide in >95% lines in minimal media (ntrB A289C). Pf0-2x, however, evolves via a number of mutations meaning the two strains diverge significantly during adaptation. We determined that this evolutionary disparity was owed to just 6 synonymous variations within the ntrB locus, which we demonstrated by swapping the sites and observing that we were able to both break (>95% to 0% in AR2) and build (0% to 80% in Pf0-2x) a powerfully deterministic mutational hotspot. Our work reveals a fundamental role for silent genetic variation in determining adaptive outcomes.

evolutionary biology↗