Ancient origin of the divergent forms of leucyl-tRNA synthetases in the Halobacteriales.
BACKGROUND
Horizontal gene transfer (HGT) has greatly impacted the genealogical history of many lineages, particularly for prokaryotes, with genes frequently moving in and out of a line of descent. Many genes that were acquired by a lineage in the past likely originated from ancestral relatives that have since gone extinct. During the course of evolution, HGT has played an essential role in the origin and dissemination of genetic and metabolic novelty.


RESULTS
Three divergent forms of leucyl-tRNA synthetase (LeuRS) exist in the archaeal order Halobacteriales, commonly known as haloarchaea. Few haloarchaeal genomes have the typical archaeal form of this enzyme and phylogenetic analysis indicates it clusters within the Euryarchaeota as expected. The majority of sequenced halobacterial genomes possess a bacterial form of LeuRS. Phylogenetic reconstruction puts this larger group of haloarchaea at the base of the bacterial domain. The most parsimonious explanation is that an ancient transfer of LeuRS took place from an organism related to the ancestor of the bacterial domain to the haloarchaea. The bacterial form of LeuRS further underwent gene duplications and/or gene transfers within the haloarchaea, with some genomes possessing two distinct types of bacterial LeuRS. The cognate tRNALeu also reveals two distinct clusters for the haloarchaea; however, these tRNALeu clusters do not coincide with the groupings found in the LeuRS tree, revealing that LeuRS evolved independently of its cognate tRNA.


CONCLUSIONS
The study of leucyl-tRNA synthetase in haloarchaea illustrates the importance of gene transfer originating in lineages that went extinct since the transfer occurred. The haloarchaeal LeuRS and tRNALeu did not co-evolve.
Department of Molecular and Cell Biology, University of Connecticut, 91 North Eagleville Rd, Storrs, CT 06269-3125, USA.
Cheryl P
CP
Andam
Timothy J
TJ
Harlow
R Thane
RT
Papke
J Peter
JP
Gogarten
D019843:Archaeal Proteins / Q000235:genetics; D019641:DNA, Archaeal / Q000235:genetics; D019143:Evolution, Molecular; D022761:Gene Transfer, Horizontal; D020745:Genome, Archaeal; D016931:Halobacteriales / Q000145:classification* / Q000201:enzymology / Q000235:genetics; D007935:Leucine-tRNA Ligase / Q000235:genetics*; D016013:Likelihood Functions; D058885:Multilocus Sequence Typing; D010802:Phylogeny; D012356:RNA, Transfer, Leu / Q000235:genetics
D019843:Archaeal Proteins; D019641:DNA, Archaeal; D012356:RNA, Transfer, Leu; D007935:Leucine-tRNA Ligase