Conversion of a bacterial warm sensor to a cold sensor by methylation of asingle residue in the presence of an attractant

The aspartate chemoreceptor (Tar) of Escherichia coli also serves as a ther mosensor, and it is very amenable to genetic and biochemical analysis of th e thermosensing mechanism. Its thermosensing properties are controlled by r eversible methylation of the cytoplasmic signalling/adaptation domain of th e protein. The unmethylated and the fully methylated (aspartate-bound) rece ptors sense, as attractant stimuli, increases (warm sensor) and decreases ( cold sensor) in temperature respectively. To learn more about the mechanism of thermosensing, we replaced the four methyl-accepting glutamyl residues with non-methylatable aspartyl residues in all possible combinations. In a strain defective in both methyltransferase (CheR) and methylesterase (CheB) activities, all of the mutant Tar proteins functioned as warm sensors. To create a situation in which all of the remaining glutamyl residues were met hylated, we expressed the mutant proteins in a CheB-defective, CheR-overpro ducing strain. The fully glutamyl-methylated proteins were designed to mimi c the full range of methylation states possible for wildtype Tar. Almost al l of the methylated mutant receptors, including those with single glutamyl residues, were cold sensors in the presence of aspartate, Thus, binding of aspartate to Tar and methylation of its single glutamyl residue can invert its temperature dependent signalling properties.