Construction and Characterization of Functional FtsA Sandwich Fusions for Studies of FtsA Localization and Dynamics during Escherichia coli Cell Division.

FtsA, a homolog of actin, is essential for cell division of Escherichia coli and is widely conserved among many bacteria. FtsA helps to tether polymers of the bacterial tubulin homolog FtsZ to the cytoplasmic membrane as part of the cytokinetic Z ring. GFP fusions to FtsA have illuminated FtsA's localization in live E. coli, but these fusions have not been fully functional and required the presence of the native FtsA. Here, we characterize "sandwich" fusions of E. coli FtsA to either mCherry or msfGFP that are functional for cell division and exhibit fluorescent rings at midcell that persist throughout constriction until cell separation. FtsA within the Z ring moved circumferentially like FtsZ, and FtsA outside the rings formed highly dynamic patches at the membrane. Notably, both FtsA-mCherry and FtsA-msfGFP acted as mild hypermorphs, as they were not toxic when overproduced, bypassed the essential cell division protein ZipA, and suppressed several thermosensitive alleles, although not as effectively as the prototypical hypermorph FtsA*. Overall, our results indicate that fluorescent FtsA sandwich fusions can be used as the sole FtsA in E. coli and thus should shed new light on FtsA dynamics during the cell division cycle in this model system. FtsA is a key conserved cell division protein, and E. coli is the most well studied model system for bacterial cell division. One obstacle to full understanding of this process is the lack of a fully functional fluorescent reporter for FtsA . Here, we describe a fluorescent fusion to E. coli FtsA that promotes efficient cell division in the absence of the native FtsA and can be used to monitor FtsA dynamics during cell division.