The suppression of signal-switching defects in the Escherichia coli serine chemoreceptor

The model bacterium Escherichia coli contains a chemotaxis system that allows the cell to change its swimming behavior in response to its environment. As the cell swims, transmembrane chemoreceptors detect concentration changes in attractant and repellent compounds and transmit signals across the inner membrane that elicit an appropriate locomotor response. Previous work identified an amino acid near the cytoplasmic tip of the receptor (F396) that plays a critical role in enabling Tsr to modulate its signal output in response to a serine stimulus. The objective of my project is to characterize second-site mutations that rescue the chemotactic ability of F396 receptor mutants. These suppressors presumably enable mutant receptors to undergo the conformational changes necessary for serine sensing and signaling. To identify such suppressors, I subjected plasmids that encode Tsr-F396G (glycine) or Tsr-F396W (tryptophan) mutant receptors to random mutagenesis and selected for mutant plasmids that promoted improved serine chemotaxis. DNA sequence analysis of the revertant plasmids showed that most of the suppressor mutations lie near the hairpin tip of Tsr. I obtained few F396W revertants and none with substantially improved function, in contrast to F396G, which gave rise to more revertants with better chemotactic ability. In subsequent experiments, I determined the expression levels of the doubly mutant Tsr proteins and characterized their signaling properties with in vitro FRET-based kinase assays. My findings served to identify Tsr structural features important for signal control of the cytoplasmic tip and enabled me to develop a more complete model of transmembrane signaling by the Tsr protein.