The RecA/RAD51 nucleoprotein filament is central to the reaction of homologous recombination (HR). residues on the concave side of RecX are important for repression of RecA activity. Analysis of RecA filament dynamics in the presence of RecX shows that RecX actively promotes filament disassembly. Collectively, our data support a model in which RecX binding to the helical groove of the filament causes local dissociation of RecA protomers, leading to filament destabilisation and HR inhibition. and may be involved in destabilising recombination intermediates (Mendonca gene is located immediately downstream of the gene. Both genes are expressed from the same cistron, which is under the transcriptional control of the LexA repressor (Pages and genes allows 5C10% transcriptional read-through of the gene, buy Celgosivir resulting in lower levels of the RecX protein relative to RecA (Pages gene did not result in a clear phenotype in gene inhibited induction of the SOS response (Pages and inhibited RecA-mediated strand exchange and ATPase activity, as well as co-protease function, at substoichiometric concentration relative to RecA (Venkatesh RecX protein and its interaction with the buy Celgosivir RecA nucleoprotein filament. The crystal structure of RecX reveals a modular architecture of tandem helical repeats, which is strongly suggestive of a mechanism of interaction with the RecA filament. We define a conserved, positively buy Celgosivir charged surface of RecX as important for its inhibitory function and test this assumption by defining RecX mutants with weakened ability to repress RecA function (2004a) suggested that RecX performs its inhibitory function by binding to the 3-end of the RecA filament, in a manner that prevents further addition of RecA protomers to the growing end of the filament. As filament disassembly continues unimpeded from the 5-end, capping’ of the 3-end by RecX would lead over time to filament dissolution. We chose to test current models of RecX function by surface plasmon resonance (SPR). Nucleoprotein filaments were generated by binding RecA to a 5-biotinylated ssDNA (60-mer oligo-dT) immobilised on a streptavidin (SA)-coated sensor chip (Figure 6A). RecA was injected into the flow cells until the binding curve indicated that maximal saturation had been achieved. We confirmed that the dynamic behaviour of RecA filaments reconstituted on the SA chip was as previously reported by examining the dependency of RecA dissociation on ATP hydrolysis (Lindsley and Cox, 1990; Arenson RecX, it is possible to identify two additional repeats in the N- and C-terminal tails of a subset of RecX sequences that exceed the consensus size by about 100 amino acids (data not shown). The existence of polypeptides spanning five repeats confirms the modular nature of the RecX protein. We identify the concave, positively charged face of the extended, curved protein shape as functionally important: mutations of conserved basic residues on the concave surface reduce the ability of RecX to buy Celgosivir inhibit RecA’s ATPase and strand-exchange activities and to induce RecA filament depolymerisation. The phylogenetic and biochemical analyses indicate that the functionally relevant portion of the RecX surface is rather extensive, suggesting an intimate association of RecX with the protein and nucleic acid components of the RecA filament. Several conserved basic residues on the concave surface of RecX participate in an extended network of cationC interactions, which constrain the basic side chains in a way that makes them poised for interaction. Involvement of functionally important side chains in a mesh of cationC interactions might reduce the entropic cost associated with formation of a intermolecular interface and might also confer additional overall rigidity to the RecX structure. By combining the crystal structure of RecX with the EM reconstruction of RecACRecX filaments, it is possible to infer the mode of RecX interaction with the filament. EM analysis of RecACdsDNA filaments stabilised by the ATP analogue AMPPNP and in the presence of functional excess of RecX revealed additional density in the helical groove of the filament. Docking of the crystallographic model of RecX into the EM reconstruction showed a good agreement between the curved, elongated shape Neurod1 of the protein and the additional, tubular density present deep in the groove of the filament. Taken together, the structural and biochemical data indicate that RecX associates with the filament by arranging its elongated shape along the bottom of the continuous filament groove, with its positively charged concave side facing towards.