Abstract
The
recJ
gene, identified in
Escherichia coli
, encodes a Mg
+2
-dependent 5′-to-3′ exonuclease with high specificity for single-strand DNA. Genetic and biochemical experiments implicate RecJ exonuclease in homologous recombination, base excision, and methyl-directed mismatch repair. Genes encoding proteins with strong similarities to RecJ have been found in every eubacterial genome sequenced to date, with the exception of
Mycoplasma
and
Mycobacterium tuberculosis
. Multiple genes encoding proteins similar to RecJ are found in some eubacteria, including
Bacillus
and
Helicobacter
, and in the archaea. Among this divergent set of sequences, seven conserved motifs emerge. We demonstrate here that amino acids within six of these motifs are essential for both the biochemical and genetic functions of
E. coli
RecJ. These motifs may define interactions with Mg
2+
ions or substrate DNA. A large family of proteins more distantly related to RecJ is present in archaea, eubacteria, and eukaryotes, including a hypothetical protein in the MgPa adhesin operon of
Mycoplasma
, a domain of putative polyA polymerases in
Synechocystis
and
Aquifex
, PRUNE of
Drosophila
, and an exopolyphosphatase (PPX1) of
Saccharomyces cereviseae
. Because these six RecJ motifs are shared between exonucleases and exopolyphosphatases, they may constitute an ancient phosphoesterase domain now found in all kingdoms of life.