Significance of two distinct types of tryptophan synthase beta chain in Bacteria, Archaea and higher plants.
BACKGROUND
Tryptophan synthase consists of two subunits, alpha and beta. Two distinct subgroups of beta chain exist. The major group (TrpEb_1) includes the well-studied beta chain of Salmonella typhimurium. The minor group of beta chain (TrpEb_2) is most frequently found in the Archaea. Most of the amino-acid residues important for catalysis are highly conserved between both TrpE subfamilies.


RESULTS
Conserved amino-acid residues of TrpEb_1 that make allosteric contact with the TrpEa subunit (the alpha chain) are absent in TrpEb_2. Representatives of Archaea, Bacteria and higher plants all exist that possess both TrpEb_1 and TrpEb_2. In those prokaryotes where two trpEb genes coexist, one is usually trpEb_1 and is adjacent to trpEa, whereas the second is trpEb_2 and is usually unlinked with other tryptophan-pathway genes.


CONCLUSIONS
TrpEb_1 is nearly always partnered with TrpEa in the tryptophan synthase reaction. However, by default at least six lineages of the Archaea are likely to use TrpEb_2 as the functional beta chain, as TrpEb_1 is absent. The six lineages show a distinctive divergence within the overall TrpEa phylogenetic tree, consistent with the lack of selection for amino-acid residues in TrpEa that are otherwise conserved for interfacing with TrpEb_1. We suggest that the standalone function of TrpEb_2 might be to catalyze the serine deaminase reaction, an established catalytic capability of tryptophan synthase beta chains. A coincident finding of interest is that the Archaea seem to use the citramalate pathway, rather than threonine deaminase (IlvA), to initiate the pathway of isoleucine biosynthesis.
Xie
Gary
G
BioScience Division, Los Alamos National Laboratory, Los Alamos, NM 87544, USA. rjensen@ufl.edu
Forst
Christian
C
Bonner
Carol
C
Jensen
Roy A
RA
D000595:Amino Acid Sequence; D001105:Archaea / Q000201:enzymology / Q000235:genetics*; D001419:Bacteria / Q000201:enzymology / Q000235:genetics*; D007527:Isoenzymes / Q000235:genetics; D008969:Molecular Sequence Data; D010802:Phylogeny; D010944:Plants / Q000201:enzymology / Q000235:genetics*; D021122:Protein Subunits; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D014367:Tryptophan Synthase / Q000235:genetics*
D007527:Isoenzymes; D021122:Protein Subunits; D014367:Tryptophan Synthase