We propose that both effects play an important role in the overall strategy of bacterial chemotaxis.
Moreover, in line with the recently described thermal robustness of the chemotaxis pathway  we observed that stability of the cluster signalling core is not affected by temperature and that the common wild type E. coli strains can perform chemotaxis up to 42°C. Results Receptor modification affects stability of the cluster core To test effects of receptor modification on the exchange dynamics of CheW and CheA at receptor clusters, FRAP experiments were performed in an adaptation-deficient (ΔcheRcheB) strain and in the CheR+ CheB+ strain. In the former strain, receptors are present in their original half-modified (QEQE) state, which leads to a nearly maximal activation of the
associated CheA in vivo [5, 8, 32]. In contrast, in the adapted CheR+ CheB+ strain the average level of receptor modification selleck chemical and activity are significantly lower [5, 8, 32, 44] (see also additional file 1, Figure S1). To facilitate FRAP experiments, both strains carried an additional deletion of the negative regulator of late flagellar and chemotaxis gene expression, anti-sigma factor FlgM. This deletion leads to an SIS3 manufacturer approximately 6-fold overexpression of all chemotaxis genes and consequently to larger clusters, without any negative effects on chemotactic performance MG132 [37, 45]. FRAP experiments were performed as previously described , whereby the fluorescence was bleached by two short laser pulses in the polar region of the cell, and subsequent recovery of relative fluorescence at the pole tuclazepam was followed over time (see Methods for details). As in this previous study, we used the C-terminal fusion of yellow fluorescent protein to CheW (CheW-YFP) and the N-terminal fusion to a truncated form of CheA that lacks first 258 amino acids (YFP-CheAΔ258). The latter fusion was chosen because it has a more clear localization pattern to receptor clusters than YFP fusion to the full-length CheA (CheAL) or the natively occurring short version of CheA (CheAS). Notably, all CheA
fusions and both N- and C-terminal CheW fusions showed similar exchange kinetics in previous FRAP experiments, suggesting that the exchange kinetics at the cluster is unaffected by the YFP fusion . Consistent with that, CheW-YFP fusion has been shown to form ultrastable ternary complexes in vitro, similar to those formed by the untagged CheW . Thus obtained recovery kinetics was clearly biphasic for all fusions (Figure 1). Our previous detailed analysis of FRAP data demonstrated that the initial phase of fast recovery corresponds to the exchange of the freely diffusing fusion protein in the region of interest, whereas the second phase specifically reflects protein exchange at the cluster .