The ability of each of the nine Escherichia coli division proteins (FtsZ, FtsA, ZipA, FtsK, FtsQ, FtsL, FtsW, FtsI, FtsN) to interact with itself and with each of the remaining eight proteins was studied in 43 possible combinations of protein pairs by the two-hybrid system previously developed by the authors' group. Once the presumed interactions between the division proteins were determined, a model showing their temporal sequence of assembly was developed. This model agrees with that developed by other authors, based on the co-localization sequence in the septum of the division proteins fused with GFP. In addition, this paper shows that the authors' assay, which has already proved to be very versatile in the study of prokaryotic and eukaryotic protein interaction, is also a powerful instrument for an in vivo study of the interaction and assembly of proteins, as in the case of septum division formation.

To clarify the function of DivIVA in Streptococcus pneumoniae, we localized this protein in exponentially growing cells by both immunofluorescence microscopy and immunoelectron microscopy and found that S. pneumoniae DivIVA (DivIVA SPN ) had a unique localization profile: it was present simultaneously both as a ring at the division septum and as dots at the cell poles. Double-immunofluorescence analysis suggested that DivIVA is recruited to the septum at a later stage than FtsZ and is retained at the poles after cell separation. All the other cell division proteins that we tested were localized in the divIVA null mutant, although the percentage of cells having constricted Z rings was significantly reduced. In agreement with its localization profile and consistent with its coiled-coil nature, DivIVA interacted with itself and with a number of known or putative S. pneumoniae cell division proteins. Finally, a missense divIVA mutant, obtained by allelic replacement, allowed us to correlate, at the molecular level, the specific interactions and some of the facets of the divIVA mutant phenotype. Taken together, the results suggest that although the possibility of a direct role in chromosome segregation cannot be ruled out, DivIVA in S. pneumoniae seems to be primarily involved in the formation and maturation of the cell poles. The localization and the interaction properties of DivIVA SPN raise the intriguing possibility that a common, MinCD-independent function evolved differently in the various host backgrounds.A number of cell division proteins have been identified in Streptococcus pneumoniae and have been shown to localize at midcell to form the septal machinery (the septosome or divisome), consistent with what is known about the best-characterized rod-shaped model organisms, Escherichia coli and Bacillus subtilis (for recent reviews, see references 12, 16, 49, and 50).These proteins include the cell division initiator proteins FtsZ and FtsA, which are required at the early stages of the process (25,29,32), and some of the later proteins, DivIB/ FtsQ, DivIC/FtsB, FtsL, FtsW, PBP 2X, and PBP 1A (29,32,33,38), which are the septal markers for S. pneumoniae cells. Recent studies have confirmed that, overall, the major events in septation are conserved in S. pneumoniae. However, other aspects related to the division process, such as the associated morphological changes, the correct choice of the division site, and proper chromosome segregation, and the factors that regulate these aspects remain largely unknown.We have described characterization of a chromosome region in S. pneumoniae, downstream of the ftsZ gene, that is well conserved among gram-positive bacteria and is physically and transcriptionally related to the division and cell wall (dcw) cluster. We showed that functional inactivation of each of the five genes in the region resulted in defects in cell morphology, chromosome segregation, and/or cell division (13), and the importance of these genes in other species has been confirmed (18,23,30). In S. pneumonia...

FtsQ, an essential protein for the Escherichia coli divisome assembly, is able to interact with various division proteins, namely FtsI, FtsL, FtsN, FtsB and FtsW. In this paper, the FtsQ domains involved in these interactions were identified by two-hybrid assays and co-immunoprecipitations. Progressive deletions of the ftsQ gene suggested that the FtsQ self-interaction and its interactions with the other proteins are localized in three periplasmic subdomains: (i) residues 50-135 constitute one of the sites involved in FtsQ, FtsI and FtsN interaction, and this site is also responsible for FtsW interaction; (ii) the FtsB interaction is localized between residues 136 and 202; and (iii) the FtsL interaction is localized at the very C-terminal extremity. In this third region, the interaction site for FtsK and also the second site for FtsQ, FtsI, FtsN interactions are located. As far as FtsW is concerned, this protein interacts with the fragment of the FtsQ periplasmic domain that spans residues 67-75. In addition, two protein subdomains, one constituted by residues 1-135 and the other from 136 to the end, are both able to complement an ftsQ null mutant. Finally, the unexpected finding that an E. coli ftsQ null mutant can be complemented, at least transiently, by the Streptococcus pneumoniae divIB/ftsQ gene product suggests a new strategy for investigating the biological significance of protein-protein interactions.

We have previously shown that integration of the virulence plasmid pINV into the chromosome of enteroinvasive Escherichia coli and of Shigella flexneri makes these strains noninvasive (C. Zagaglia, M. Casalino, B. Colonna, C. Conti, A. Calconi, and M. Nicoletti, Infect. Immun. 59:792-799, 1991). In this work, we have studied the transcription of the virulence regulatory genes virB, virF, and hns (virR) in wild-type enteroinvasive E. coli HN280 and in its pINV-integrated derivative HN280/32. While transcription of virF and of hns is not affected by pINV integration, transcription of virB is severely reduced even if integration does not occur within the virB locus. This indicates that VirF cannot activate virB transcription when pINV is integrated, and this lack of expression accounts for the noninvasive phenotype of HN280/32. Virulence gene expression in strains HN280 and HN280/32, as well as in derivatives harboring a mxiC::lacZ operon fusion either on the autonomously replicating pINV or on the integrated pINV, was studied. The effect of the introduction of plasmids carrying virB (pBN1) or virF (pHW745 and pMYSH6504), and of a ⌬hns deletion, in the different strains was evaluated by measuring ␤-galactosidase activity, virB transcription, and virB-regulated virulence phenotypes like synthesis of Ipa proteins, contactmediated hemolysis, and capacity to invade HeLa cells. The introduction of pBN1 or of the ⌬hns deletion in pINV-integrated strains induces temperature-regulated expression or temperature-independent expression, respectively, of ␤-galactosidase activity and of all virulence phenotypes, while an increase in virF gene dosage does not, in spite of a high-level induction of virB transcription. Moreover, a wild-type hns gene placed in trans fully reversed the induction of ␤-galactosidase activity due to the ⌬hns deletion. These results indicate that virB transcription is negatively regulated by H-NS both at 30 and at 37؇C in pINV-integrated strains and that there is also a dosedependent effect of VirF on virB transcription. The negative effect of H-NS on virB transcription at the permissive temperature of 37؇C could be due to changes in the DNA topology occurring upon pINV integration that favor more stable binding of H-NS to the virB promoter DNA region. At 30؇C, the introduction of the high-copy-number plasmid pMYSH6504 (but not of the low-copy-number pHW745) or of the ⌬hns deletion induces, in strains harboring an autonomously replicating pINV, ␤-galactosidase activity, virB transcription, and expression of the virulence phenotypes, indicating that, as for HN280/32, the increase in virF gene dosage overcomes the negative regulatory effect of H-NS on virB transcription. Moreover, we have found that virF transcription is finely modulated by temperature and, with E. coli K-12 strains containing a virF-lacZ gene fusion, by H-NS. This leads us to speculate that, in enteroinvasive bacteria, the level of VirF inside the cell controls the temperature-regulated expression of invasion genes.Shigella flexneri and ...

The development of a convenient and promising alternative to the various two-hybrid methods that are used to study protein-protein interactions is described. In this system, a lambdoid chimeric operator is recognized by a hybrid repressor formed by two chimeric monomers whose C-terminal domains are composed of heterologous proteins (or protein domains). Only if these proteins efficiently dimerize in vivo is a functional repressor formed able to bind the chimeric operator and shut off the synthesis of a downstream reporter gene. This new approach was tested with several interacting proteins ranging in size from less than 100 to more than 800 amino acids and, to date, no size or topology limit has been detected.