DNA-binding protein Fis <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"/>]. It activates ribosomal RNA transcription, and is involved in upstream activation of rRNA promoters. Found in gamma proteobacterial microbes, the protein has been shown to play a part 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="PUB00007181"/>]. Some of its functions include inhibition of the initiation of DNA replication from the OriC site, and promotion of Hin-mediated DNA inversion [<cite idref="PUB00007182"/>].</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> Recently, the crystal structure of wild-type E. coli FIS was resolved, together with six mutants [<cite idref="PUB00007184"/>] - the first crystal structure was solved in 1991. From the most recent 2.0A structure [<cite idref="PUB00007786"/>] of wild-type FIS, the protein was observed to exist as a homodimer in the bacterial cytoplasm. By comparison with the structures of FIS mutants, it was deduced that arginine-71 is critical for the binding of FIS to RNA polymerase, while glycine-72 stabilises the tertiary structure.</p>