The role of the carboxy terminus of the Escherichia coli cell division protein FtsA in bacterial division has been studied by making a series of short sequential deletions spanning from residue 394 to 420. Deletions as short as 5 residues destroy the biological function of the protein. Residue W415 is essential for the localization of the protein into septal rings. Overexpression of the ftsA alleles harboring these deletions caused a coiled cell phenotype previously described for another carboxy-terminal mutation (Gayda et al., J. Bacteriol. 174:5362-5370, 1992), suggesting that an interaction of FtsA with itself might play a role in its function. The existence of such an interaction was demonstrated using the yeast two-hybrid system and a protein overlay assay. Even these short deletions are sufficient for impairing the interaction of the truncated FtsA forms with the wild-type protein in the yeast two-hybrid system. The existence of additional interactions between FtsA molecules, involving other domains, can be postulated from the interaction properties shown by the FtsA deletion mutant forms, because although unable to interact with the wild-type and with FtsADelta1, they can interact with themselves and cross-interact with each other. The secondary structures of an extensive deletion, FtsADelta27, and the wild-type protein are indistinguishable when analyzed by Fourier transform infrared spectroscopy, and moreover, FtsADelta27 retains the ability to bind ATP. These results indicate that deletion of the carboxy-terminal 27 residues does not alter substantially the structure of the protein and suggest that the loss of biological function of the carboxy-terminal deletion mutants might be related to the modification of their interacting properties.