Second-site suppression analysis

CheZ is a unique protein that, of the known bacterial chemotaxis systems and two-component regulatory systems, exists only in chemo-tactic enteric bacteria such as E. coli and Salmonella [494] as well as in P. aeruginosa [339, 709]. Its role is to enhance CheY P dephosphorylation [293]. (The term phosphatase is used in this chapter to describe, in the broader sense, the activity of CheZ. It does not connote the mechanism of CheZ action.) CheZ does so with a high specificity, as evident from its failure to dephosphorylate CheB [293], even though the N-terminal portion of the latter is homologous to CheY as a whole [696]. Initially it was assumed, on the basis of second-site suppression analysis in which mutations in cheZ were phenotypically suppressed by... 

The chemotaxis protein CheY was subsequently identified by both the whole-cell and reductionist approaches as the clockwise signal, i.e., as the molecule that interacts with the switch, thereby promoting clockwise rotation. In the whole-cell approach, second-site suppression analysis implied that CheY interacts with the switch (for reviews, see [56, 459]), and overproduction of CheY in a gutted strain generated clockwise rotation ([168] for a review, see reference [212]). In the reductionist approach, purified CheY inserted into cytoplasm-free envelopes caused some of the envelopes to rotate clockwise [594]. The absence of cytoplasmic chemotaxis proteins in the intact cells and the absence of cytoplasm in the envelopes indicated that the interaction of CheY with the switch is direct. Later biochemical studies demonstrated... 

Microkinetic analysis suggests two possible ways to lower the carbon threshold. The first is to lower the surface carbon site coverage, resulting also in lower activity to syngas formation. The second is to increase the filamentous carbon solubility in Ni and thus lower the driving force for carbon deposition. This means that smaller crystal sizes of Ni will suppress filamentous carbon deposition without lowering the activity for syngas formation. 

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