The Escherichia coli gene firA, previously reported to code for a small, histonelike DNA-binding protein, has been cloned and found to reside immediately downstream from skp, a gene previously identified as the firA locus. firA encodes a 36-kDa protein. The mutant firA200(Ts) allele was also cloned and shown to contain three mutations, each mutation giving rise to a single amino acid change. Partially purified wild-type FirA (from a firA+ strain) and mutant FirA [from a firA200(Ts) strain] proteins have amino-terminal sequences predicted from their common DNA sequences. Both proteins lack an N-terminal methionine. Modest overexpression of wild-type or mutant FirA restored wild-type growth to firA200(Ts) strains at 43 degrees C, whereas high-level expression of wild-type FirA was required for more complete suppression of the rifampin sensitivity of firA200(Ts) rpoB double mutants. High-level expression of mutant FirA did not suppress this rifampin sensitivity.