Team:Paris/Flagella

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Description of the genetic pathway that lead to flagella assembly

Figure 1 : The genetically defined hierarchy of the flagellar operons in Escherichia coli.

The construction of E. coli flagellar system is organized into a complex hierarchy. The different parts of the flagella are successively assembled parts by parts. This architecture is also present in the network of genes that contributes to its set-up.

A global transcription factor FlhDC activates the synthesis of the flagella. This class 1 gene encodes a protein that activates the expression of class 2 genes. Those genes lead to the construction of the basal body of the flagella and its hook. Then FliA, a protein coded by a class 2 gene activates, with FlhDC to the expression of class 3 genes (Figure 1).


The expression of the flagellar genes is precisely ordered

Figure 2 : The sequential expression of flagellar genes
When the flagella system is turned on, the genes are expressed sequentially. flhD is expressed first, then the class 2 genes, and lastly the class 3 genes. At the heart of each class, the promoters are turned on sequentially, with significant delays, in the order fliL, fliE, fliF, flgA, flgB, flhB, and fliA (Figure 2).

The observed order correponds to the spatial position of the gene products during flafellar motor assemblu, going from the cytoplasmic to the extracellular sides.

A quantitative blueprint of the dynamics of the flagella gene network

Blueprint.png
Kalir et al. (2004) evaluated the strength of the interactions between the different transcription factors fliA and flhDC and the class 2 promoters. They established a quantitative blueprint, considering the strength of the interactions and the functions that integrate multiple input (Figure 3).

They determined that the input function for fliL promoter is a SUM gate in relation to FliA and FlhDC.
It means that  :

Promoter_Activity([FlhDC],[FliA]) = Promoter_Activity([FlhDC],0) + Promoter_Activity(0,[FliA])

Bibliography