Lambda – E. coli co-evolution
Bryan Andrews
Phages and bacteria engage in an evolutionary arms race centered at the interface between the phage tail fiber and the bacterial receptor. Bacterial phage-resistance variants arise quickly in phage-exposed populations, typically with alterations in, or loss of, surface receptors. Consequently, phages must rapidly adapt to use new receptors, either by restoring affinity to mutated receptors or by switching to orthologous receptors. Understanding these co-evolutionary dynamics will be critical in developing phage-based therapeutic applications.
To study this process, we use the well-studied systems of E. coli and phage λ, for which the relevant proteins are the E. coli surface receptor maltoporin and the λ tail fiber, J protein. We generated libraries of J containing a large fraction of the single-residue substitutions over a 150-residue domain known to interact with the bacterial surface. We selected for functional variants by allowing the library to infect E. coli over several generations in conditions that favor rapid binding and we sequenced the population at each generation to determine variant fitnesses.
We discovered that missense substitutions in J are, in general, highly deleterious compared to other substitutions in other proteins that have been similarly assayed. However, the sequence is highly patterned, with individual residues displaying a wide array of tolerance or intolerance to mutations. We are currently working to distinguish substitutions that are generally destabilizing from substitutions that specifically abrogate binding to the receptor and/or promote binding to novel receptors. We are also pursuing structural work to determine how the residues we have assayed are spatially related to the receptor.
To study this process, we use the well-studied systems of E. coli and phage λ, for which the relevant proteins are the E. coli surface receptor maltoporin and the λ tail fiber, J protein. We generated libraries of J containing a large fraction of the single-residue substitutions over a 150-residue domain known to interact with the bacterial surface. We selected for functional variants by allowing the library to infect E. coli over several generations in conditions that favor rapid binding and we sequenced the population at each generation to determine variant fitnesses.
We discovered that missense substitutions in J are, in general, highly deleterious compared to other substitutions in other proteins that have been similarly assayed. However, the sequence is highly patterned, with individual residues displaying a wide array of tolerance or intolerance to mutations. We are currently working to distinguish substitutions that are generally destabilizing from substitutions that specifically abrogate binding to the receptor and/or promote binding to novel receptors. We are also pursuing structural work to determine how the residues we have assayed are spatially related to the receptor.
