The lac operon is a classic model system for bacterial gene regulation, and has been studied extensively in E. coli, a classic model organism. However, not much is known about E. coli’s ecology and life outside the laboratory, in particular in soil and water environments. The natural diversity of the lac operon outside the laboratory, its role in the ecology of E. coli and the selection pressures it is exposed to, are similarly unknown. In Chapter Two of this thesis, I explore the genetic diversity, phylogenetic history and signatures of selection of the lac operon across 20 natural isolates of E. coli and divergent clades of Escherichia. I found that complete lac operons were present in all isolates examined, which in all but one case were functional. The lac operon phylogeny conformed to the whole-genome phylogeny of the divergent Escherichia clades, which excludes horizontal gene transfer as an explanation for the presence of functional lac operons in these clades. All lac operon genes showed a signature of purifying selection; this signature was strongest for the lacY gene. Lac operon genes of human and environmental isolates showed similar signatures of selection, except the lacZ gene, which showed a stronger signature of selection in environmental isolates. In Chapter Three, I try to identify the natural genetic variation relevant for phenotype and fitness in the lac operon, comparing growth rate on lactose and LacZ activity of the lac operons of these wild isolates in a common genetic background. Sequence variation in the lac promoter region, upstream of the -10 and -35 RNA polymerase binding motif, predicted variation in LacZ activity at full induction, using a thermodynamic model of polymerase binding (Tugrul, 2016). However, neither variation in LacZ activity, nor RNA polymerase binding predicted by the model correlated with variation in growth rate. Lac operons of human and environmental isolates did not differ systematically in either growth rate on lactose or LacZ protein activity, suggesting that these lac operons have been exposed to similar selection pressures. We thus have no evidence that the phenotypic variation we measured is relevant for fitness. To start assessing the effect of genomic background on the growth phenotype conferred by the lac operon, I compared growth on minimal medium with lactose between lac operon constructs and the corresponding original isolates, I found that maximal growth rate was determined by genomic background, with almost all backgrounds conferring higher growth rates than lab strain K12 MG1655. However, I found no evidence that the lactose concentration at which growth was half maximal depended on genomic background.
ERC H2020 programme (grant agreement no. 648440) Thanks to Jon Bollback for giving me the chance to do this work, for sharing the ideas that lay at the basis of this work, for his honesty and openness, showing himself to me as a person and not just as a boss. Thanks to Nick Barton for his guidance at the last stage, reading and commenting extensively on several versions of this manuscript, and for his encouragement; thanks to both Jon and Nick for their kindness and patience. Thanks to Erik van Nimwegen and Calin Guet for their time and willingness to be in my thesis committee, and to Erik van Nimwegen especially for agreeing to enter my thesis committee at the last moment, and for his very sharp, helpful and relevant comments during and after the defense. Thanks to my collaborators and discussion partners: Anne Kupczok, for her guidance, ideas and discussions during the construction of the manuscript of Chapter Two, and her comments on the manuscript; Georg Rieckh for making me aware of the issue of parameter identifiability, suggesting how to solve it, and for his unfortunate idea to start the plasmid enterprise in the first place; Murat Tugrul for sharing his model, for his enthusiasm, and his comments on Chapter Three; Srdjan Sarikas for his collaboration on the Monod model fitting, fast forwarding the analysis to turbo speed and making beautiful figures, and making the discussion fun on top of it all; Vanessa Barone for her last minute comments, especially on Chapter Three, providing a sharp and very helpful experimentalist perspective at the last moment; Maros Pleska and Marjon de Vos for their comments on the manuscript of Chapter Two; Gasper Tkacik for his crucial input on the relation between growth rate and lactose concentration; Bor Kavcic for his input on growth rate modeling and error propagation. Thanks to the Bollback, Bollenbach, Barton, Guet and Tkacik group members for both pro- viding an inspiring and supportive scientific environment to work in, as well as a lot of warmth and colour to everyday life. And thanks to the friends I found here, to the people who were there for me and to the people who changed my life, making it stranger and more beautiful than I could have imagined, Maros, Vanessa, Tade, Suzi, Andrej, Peter, Tiago, Kristof, Karin, Irene, Misha, Mato, Guillaume and Zanin.
Jesse F. The lac operon in the wild. 2017. doi:10.15479/AT:ISTA:th_857
Jesse, F. (2017). The lac operon in the wild. Institute of Science and Technology Austria. https://doi.org/10.15479/AT:ISTA:th_857
Jesse, Fabienne. “The Lac Operon in the Wild.” Institute of Science and Technology Austria, 2017. https://doi.org/10.15479/AT:ISTA:th_857.
F. Jesse, “The lac operon in the wild,” Institute of Science and Technology Austria, 2017.
Jesse F. 2017. The lac operon in the wild. Institute of Science and Technology Austria.
Jesse, Fabienne. The Lac Operon in the Wild. Institute of Science and Technology Austria, 2017, doi:10.15479/AT:ISTA:th_857.
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