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Escherichia coli has proteins that are specialised in forming the division septum
Many of them were identified by means of mutants in which genetic lesions inactivated the function of one class of protein under specific conditions. Most mutations produced a useless protein at a temperature higher than the normal one (37ºC is the preferred temperature for an inhabitant of the human gut).Division proteins assemble at the center of the cell to initiate and complete the building of the septumWhen transferred from 30 to 42ºC the division mutants continue growing, but they are not able to divide. As a result the cells elongate ceaselessly and form long filaments. These mutations are consequently named fts, an acronym for filamentous temperature sensitive.
Division proteins are found at different locations but they assemble in the middle of the cell during the process of division, where some of them form a division ring.
Many genes that code for cell division proteins in Escherichia coli are grouped in a gene clusterSome division proteins, FtsZ is the best known example, are dispersed in the cytoplasm (the interior of the cell) at all times during the bacterial life until septation is triggered. At this moment the FtsZ molecules join together in the middle of the cell where they form a ring. Additional proteins, like FtsA, are later recruited into the ring and remain there until division is completed.
We still do not know what makes the FtsZ molecules migrate to the cell center and become associated to the membrane at a specific time during the cell cycle.
There are wide differences in the number of molecules of each Fts protein per cell, from the abundant FtsZ (5000 to 10000) to the scarce FtsQ (less than 50), but all are needed for the Escherichia coli cell to divide.
The genes that code for a dozen proteins involved either in cell division (FtsA, Q, W, and Z among them) or cell wall biosynthesis (for example FtsI) are located together in the same region of the chromosome. This is called the dcw cluster, and it is highly conserved in many distantly related bacteria.
We do not know for certain why these genes are grouped together in many different bacteria, perhaps their grouping is favourable if it allows them to be regulated as a block under some conditions in which division needs to be strictly controlled to allow the survival of the cell.
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last update: 22 January 2003