nzymes (NISE) is the preferred term to accurately describe enzymes that lack detectable sequence similarity but catalyze the same biochemical reactions and carry the same Enzyme Classification (EC) number 
. NISE have previously been referred to as analogous enzymes 
. In many cases, NISE also lack structural similarity, this being a more robust indicator of independent evolutionary routes towards fulfilling a common metabolic conversion 
. NISE most likely evolve by recruitment of existing enzymes that take on a new cellular function following changes to the substrate binding site or catalytic mechanism. This scenario is most plausible when one or both members of a pair of NISE belong to a larger enzyme family that catalyzes related reactions. For example, gluconate kinase from Bacillus subtilis
has orthologs within the genus Bacillus
but is otherwise unrelated to gluconate kinases from other bacteria or eukaryotes. However, the Bacillus
enzyme belongs to a larger kinase family that includes xylulose kinase and glycerol kinase in other taxa. A duplication in the gene encoding either xylulose kinase or glycerol kinase is presumed to have occurred in the lineage leading to the Bacilli and been followed by a shift in substrate specificity to generate the novel gluconate kinase 
. Lateral gene transfer (LGT) events can further shape the distribution of NISE in different taxonomic groups and introduce enzyme activities analogous to ones already encoded by the recipient genome. The protozoan parasite, Trichomonas vaginalis
, for example, encodes distinct forms of malic enzymes, one of which appears to be the result of LGT from a eubacterium 
. The combination of enzyme recruitments and LGTs coupled with independent gene losses and gene gains in different lineages can therefore lead to patchy distributions of NISE forms when viewed across broad phylogenetic distances.
Phosphoglycerate mutase (PGM; E.C. 18.104.22.168.) catalyzes the interconversion of 2- and 3-phosphoglycerate (2-PG and 3-PG) in the glycolytic and gluconeogenic pathways. Two distinct forms of PGM that have no similarity in protein size, primary sequence, three-dimensional structure or catalytic mechanism are known to exist and are considered analogous enzymes (NISE) 
. One form, cofactor-dependent PGM (dPGM), requires the cofactor 2,3-bisphosphoglycerate (2,3-BPG) for activity. The dPGM enzymes, having a molecular mass of about 27 kD, are usually active as dimers or tetramers and catalyze the intermolecular transfer of a phosphoryl group between the monophosphoglycerates and the cofactor via a phosphohistidine intermediate. Sequence and structural analyses of dPGM enzymes place them in the acid phosphatase superfamily along with enzymes such as fructose-2,6-bisphosphatase and acid phosphatase 
. On the other hand, cofactor-independent PGM (iPGM) is typically about 57 kD, active as a monomer, and catalyzes the intramolecular transfer of the phosphoryl group between monophosphoglycerates through a phosphoserine intermediate. The iPGM enzymes belong to the alkaline phosphatase superfamily along with enzymes such as phosphopentomutases and certain sulfatases to name a few 
. The two forms of PGM can be distinguished further by the metal ion requirement of iPGM and the sensitivity of dPGM to vanadate 
PGM sequences, in particular those of iPGM, appear to be evolving very slowly 
and are generally very well conserved even across different kingdoms 
, allowing their identification in genome sequences from diverse organisms. However, since both dPGM and iPGM are members of larger phosphatase superfamilies containing diverse enzymes with related sequences, the identification of PGMs solely by sequence similarity should be treated with caution. Indeed, a predicted dPGM of Bacillus
spp. was subsequently shown by molecular modeling and enzymatic analyses of recombinant protein to encode a broad specificity phosphatase 
. Small-scale bioinformatic surveys and biochemical studies have indicated that only iPGM is present in plants and nematodes while only dPGM is found in mammals 
. However, within other phylogenetic groups the distribution of the two PGM forms is complex and has been described as appearing haphazard 
. Most bacteria, archaea, protozoa and fungi contain either iPGM or dPGM, while some bacteria such as Escherichia coli
and certain archaea and protozoa contain both forms. The respective roles of dPGM and iPGM in organisms that contain both forms of enzyme are uncertain.
In E. coli,
at least, distinct PGM activities were reported for both dPGM and iPGM in crude cell extracts and when expressed in recombinant form 
. The dPGM form accounted for the great majority of activity leaving unanswered questions about the role of iPGM in E. coli
. To gain insight into the respective functions of dPGM and iPGM in E.coli
, we generated null mutants for phenotypic studies to examine the role of each enzyme. We report that loss of dPGM leads to delayed growth both in liquid cultures and on solid medium, apparently due to a delay or defect in exiting stationary phase. We further show that the wild type phenotype can be restored by overexpression of either dPGM or iPGM in dPGM
null mutants. We also produced recombinant dPGM and iPGM for detailed biochemical analyses to address the specific PGM and phosphatase activities of each enzyme. We demonstrate that the distinct PGM forms present in E.coli
have overlapping and complementary roles in the cell.
The evolutionary origins of dPGM and iPGM that underlie the unpredictable distribution of these NISE proteins in bacteria are not clear 
. However, the abundance of sequenced microbial genomes provides an unprecedented opportunity to address the distribution of NISE across hundreds of bacterial species. In the present study we performed a comprehensive survey of the distribution of the PGM forms throughout the bacterial domain to gain insight into the processes and events that appear to have contributed to their apparently haphazard phyletic profiles.