Table 1.

Overall structural characteristics of aminoacyl-tRNA synthetases, with emphasis on E. coli members

Class I aaRSs			Class II aaRSs
Subclasses	Oligomeric structure	No. aa/subunit (E. coli aaRSs)	Subclasses	Oligomeric structure	No. aa/subunit (E. coli aaRSs)
Class Ia			Class IIa
LeuRS	α or αβa	860	SerRS	α2	430
IleRSl	α	937b	ThrRS	α2	642
ValRS	α	951	ProRS	α2	572 or 590
ArgRS	α	577	HisRS	α2	424
CysRS	α	461	GlyRS	α2β2c	303 & 689
MetRS	α2 or α	642
Class Ib			Class IIb
GluRS	αd	471	AspRS	α2	590
GlnRS	α	551	AsnRS	α2	467
LysRS I e	α	–f	LysRS	α2	505
Class Ic			Class IIc
TyrRS	α2	424	AlaRS	α4	875
TrpRS	α2	334	PheRSg	α2β2h	327 & 795
			PylRSi	α2	–j
			SepRSh	α4	–k

^a Can be αβ heterodimeric in a few Bacteria, such as Aquifex aeolicus from the phylum Aquificae (see “Structures of aminoacyl-tRNA synthetases,” below).

^b Revised amino acid sequence by means of mass spectrometry peptide mapping (42).

^c Tetrameric α₂β₂ GlyRSs are exclusively found in Bacteria; however, a few Bacteria contain dimeric α₂ GlyRSs (of archaeal/eukaryal-type). A dimeric α₂-type GlyRS in Eukarya, was as first found in baker’s yeast (43).

^d A catalytically active homodimeric form of Mycobacterium tuberculosis GluRS has been characterized that is in equilibrium with the monomer (44).

^e Present in a few Bacteria, but mainly in Archaea.

^f 523 amino acids in archaeal Pyrococcus horikoshii (39).

^g Small catalytic α-subunit is the class II characteristic domain in PheRSs.

^h Monomers in mitochondria.

ⁱ Noncanonical archaeal aaRSs (PylRS and SepRS); note the presence of PylRS in some Bacteria, such as in Desulfitobacteria hafniense (46) from the phylum Firmicutes.

^j 454 amino acids in the subunit of the Methanococcus mazei PylRS (47).

^k 534 amino acids in the subunits of the Archaeoglobus fulgidus SepRS (48, 49).

^l Underlined aaRSs: protein sequence available.

For additional references on aaRS sequences, see reference 10.