<p>The Factor for Inversion Stimulation (FIS) protein is a regulator of bacterial functions, and binds specifically to weakly related DNA sequences [<cite idref="PUB00007180"/>,<cite idref="PUB00007181"/>]. It activates ribosomal RNA transcription, and is involved in upstream activation of rRNA promoters. The protein has been shown to play a role in the regulation of virulence factors in both <taxon tax_id="602">Salmonella typhimurium</taxon> and <taxon tax_id="562">Escherichia coli</taxon> [<cite idref="PUB00007182"/>]. Some of its functions include inhibition of the initiation of DNA replication from the OriC site, and promotion of Hin-mediated DNA inversion.</p> <p> In its C-terminal extremity, FIS encodes a helix-turn-helix (HTH) DNA- binding motif, which shares a high degree of similarity with other HTH motifs of more primitive bacterial transcriptional regulators, such as the nitrogen assimilation regulatory proteins (NtrC) from species like Azobacter, Rhodobacter and Rhizobium. This has led to speculation that both evolved from a single common ancestor [<cite idref="PUB00007183"/>].</p><p>The 3-dimensional structure of the E. coli FIS DNA-binding protein has been determined by means of X-ray diffraction to 2.0A resolution [<cite idref="PUB00005701"/>,<cite idref="PUB00007184"/>]. FIS is composed of four alpha-helices tightly intertwined to form a globular dimer with two protruding HTH motifs. The 24 N-terminal amino acids are poorly defined, indicating that they might act as `feelers' suitable for DNA or protein (invertase) recognition [<cite idref="PUB00005701"/>]. Other proteins belonging to this subfamily include:</p><ul> <li>E. coli: atoC, hydG, ntrC, fhlA, tyrR,</li><li> Rhizobium spp.: ntrC, nifA, dctD</li></ul> Helix-turn-helix, Fis