A gain-of-function mutation in ftsA bypasses the requirement for the essential cell division gene zipA in Escherichia coli

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Abstract
ZipA and FtsA are recruited independently to the FtsZ cytokinetic ring (Z ring) and are essential for cell division of Escherichia coli . The molecular role of FtsA in cell division is unknown; however, ZipA is thought to stabilize the Z ring, anchor it to the membrane, and recruit downstream cell division proteins. Here we demonstrate that the requirement for ZipA can be bypassed completely by a single alteration in a conserved residue of FtsA (FtsA*). Cells with ftsA * in single copy in place of WT ftsA or with ftsA * alone on a multicopy plasmid divide mostly normally, whether they are zipA + or zipA −. Experiments with ftsQAZ and ftsQA * Z on multicopy plasmids indicate that ftsQAZ / zipA + and ftsQA * Z / zipA − cells divide fairly normally, whereas ftsQAZ / zipA − cells divide poorly and ftsQA * Z / zipA + cells display a phenotype that suggests their septa are unusually stable. In support of the idea that ftsA * stabilizes Z rings, single-copy ftsA * confers resistance to excess MinC, which destabilizes Z rings. The inhibitory effect of excess ZipA on division is also suppressed by ftsA *. These results suggest that the molecular mechanism of the FtsA* bypass is to stabilize FtsZ assembly via a parallel pathway and that FtsA* can replace the multiple functions of ZipA. This is an example of a complete functional replacement of an essential prokaryotic cell division protein by another and may explain why most bacteria can divide without an obvious ZipA homolog.