The Role of Genomic Context in Bacterial Growth Homeostasis
The growth of bacteria is a complex but well-orchestrated dance involving the repetitive and reproducible production of their diverse cellular components in order to divide. A lot can go astray and therefore the cell has developed several strategies in order to ensure everything remains synchronized...
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|The growth of bacteria is a complex but well-orchestrated dance involving the repetitive and reproducible production of their diverse cellular components in order to divide. A lot can go astray and therefore the cell has developed several strategies in order to ensure everything remains synchronized. This problem is only further complicated as the cells adjust their growth rate to their living conditions resulting in ripple effects throughout the cell physiology. One notable change is that as nutrient availability and quality increases so too does the average size and the concentration of ribosomes in the cell. The latter enables the production of the largest macromolecule faction in the cell (proteins) including the production of more ribosomes required to maintain the protein synthesis requirements. With the increase in volume of the cell comes a required increase in surface area, and a disbalance between these two would result in untenable levels of internal pressure. How then do bacteria ensure that volume growth is synchronized with the production of cell envelope components so that cell homeostasis is maintained, especially in the face of fluctuating growth rate? Genomic context is known to assist in co-regulation of genes thereby synchronizing them to respond to different cellular stimuli. As the bacterial genome is highly fluid, the existence of conserved genomic contexts suggests important loci of co-regulation. Could it be in these gene clusters that a possible link between growth and surface expansion is found?
To answer this question this thesis undertook three missions, firstly we established a genome comparison tool (www.GenCoDB.org) that will take advantage of the ever-growing availability of bacterial genomes to assist us in the analysis, comparison, and quantification of genome contexts. This will rely on novel strategies in order to: accommodate the breadth of genome data available in a computationally efficient manner, reduce the effect of sampling bias that plague most bacterial datasets and ensure candidates are considered significant for their evolutionary context. The availability of GenCoDB is sure to facilitate genomic context research in the microbiology community and improve accessibility to non-bioinformatics to this wellspring of important biological data.
With the swath of genomic neighbourhood data, we then sought to understand and analyse the evolution of conserved gene clusters in order to narrow down possible volume-surface regulating candidates. By tracking the evolution of gene clusters throughout the Bacteria kingdom we found that co-orientation is strongly conserved, however, this does not influence the subsequent context around the cluster nor the expansion of the cluster. We found that vertical transmission and not horizontal gene transfer was found to be the driving factor of gene cluster occurrence in chromosomes and that the origin and terminus are hotspots for cluster maintenance. Finally, we found that despite the apparent frequency of operon organization in gene clusters, gene clusters appear to be maintained due to other selective pressures such as within-cluster protein-protein interactions and the essential status of their genes. We suggest that operons are a consequence and not a cause co-localization over evolutionary time.
We identified a single gene cluster candidate that met all the requirements we believe are required for cell growth homeostasis of synchronized surface and volume expansion. These requirements were a broad conservation within Bacteria, and a connection between ribosome-associated proteins (growth) with cell envelope synthesizes. In agreement with our evolution studies we found that whilst the cluster was co-regulated this did not appear to be the selective pressure that brought these different processes together. Instead we found a potential role of genomic channelling, linking the production of pyrimidines with the synthesis of the cell envelope which is reliant on the co-localization of this cluster.
Together, this work will forward the understanding of chromosome evolution in Bacteria and the potential implications of genomic context in metabolite utilization. It challenges the roles that operons and horizontal gene transfer play in the long-term evolution of gene order and it provides a new quantitative and statistical resource providing access to over 1.9 million gene neighbourhoods.