What is known about the structure and function of the Escherichia coli protein FirA?

The firA gene is essential for growth of Escherichia coli growth and lies in the 4-minute region of the genome. firA encodes the FirA protein which contains 341 amino acids and has an apparent molecular mass of 36 kDa. Genetic evidence suggests that FirA plays a role in transcription since certain firA alleles confer temperature sensitivity for growth and RNA synthesis as well as reversing the rifampin resistance of rifampin-resistant rpoB mutants ('fir' effect). FirA co-immunoprecipitates with RNA polymerase holoenzyme, implying a physical association with the transcriptional machinery, possibly with the beta subunit of RNA polymerase. FirA contains a previously undescribed isoleucine/valine-rich six-amino-acid repeat occurring 14 times within the N-terminal and 12 more times within the C-terminal half of the protein. This repeat can be formulated as [HXXXhZ]n with 'H' representing a large non-polar residue (usually isoleucine), 'h' representing a smaller non-polar residue, 'Z' representing either charged/polar or aromatic residues, XXX representing residues typical of beta turns, and 'n' being equal to the repeat number. We refer to this repeat as an isoleucine patch. Proteins encoded by three E. coli acyltransferases also contain this motif which is roughly positioned in each case, within the amino- and carboxyl termini, as in FirA. When the sequences of these proteins are aligned, a region of poor similarity separates the isoleucine patches. The significance of these repeats remains unknown although we speculate that they play an important structural role in the organization and function of FirA (and other proteins containing isoleucine patches), possibly by acting as homo- or hetero-dimerization interfaces.(ABSTRACT TRUNCATED AT 250 WORDS)