An ancient family of SelB elongation factor-like proteins with a broad but disjunct distribution across archaea.
BACKGROUND
SelB is the dedicated elongation factor for delivery of selenocysteinyl-tRNA to the ribosome. In archaea, only a subset of methanogens utilizes selenocysteine and encodes archaeal SelB (aSelB). A SelB-like (aSelBL) homolog has previously been identified in an archaeon that does not encode selenosysteine, and has been proposed to be a pyrrolysyl-tRNA-specific elongation factor (EF-Pyl). However, elongation factor EF-Tu is capable of binding archaeal Pyl-tRNA in bacteria, suggesting the archaeal ortholog EF1A may also be capable of delivering Pyl-tRNA to the ribosome without the need of a specialized factor.


RESULTS
We have phylogenetically characterized the aSelB and aSelBL families in archaea. We find the distribution of aSelBL to be wider than both selenocysteine and pyrrolysine usage. The aSelBLs also lack the carboxy terminal domain usually involved in recognition of the selenocysteine insertion sequence in the target mRNA. While most aSelBL-encoding archaea are methanogenic Euryarchaea, we also find aSelBL representatives in Sulfolobales and Thermoproteales of Crenarchaea, and in the recently identified phylum Thaumarchaea, suggesting that aSelBL evolution has involved horizontal gene transfer and/or parallel loss. Severe disruption of the GTPase domain suggests that some family members may employ a hitherto unknown mechanism of nucleotide hydrolysis, or have lost their GTPase ability altogether. However, patterns of sequence conservation indicate that aSelBL is still capable of binding the ribosome and aminoacyl-tRNA.


CONCLUSIONS
Although it is closely related to SelB, aSelBL appears unlikely to either bind selenocysteinyl-tRNA or function as a classical GTP hydrolyzing elongation factor. We propose that following duplication of aSelB, the resultant aSelBL was recruited for binding another aminoacyl-tRNA. In bacteria, aminoacylation with selenocysteine is essential for efficient thermodynamic coupling of SelB binding to tRNA and GTP. Therefore, change in tRNA specificity of aSelBL could have disrupted its GTPase cycle, leading to relaxation of selective pressure on the GTPase domain and explaining its apparent degradation. While the specific role of aSelBL is yet to be experimentally tested, its broad phylogenetic distribution, surpassing that of aSelB, indicates its importance.
University of Tartu, Institute of Technology, Tartu, Estonia. gemma.atkinson@ut.ee
Gemma C
GC
Atkinson
Vasili
V
Hauryliuk
Tanel
T
Tenson
D000595:Amino Acid Sequence; D001105:Archaea / Q000737:chemistry / Q000145:classification / Q000235:genetics* / Q000378:metabolism; D019843:Archaeal Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D001665:Binding Sites; D008969:Molecular Sequence Data; D005810:Multigene Family; D010445:Peptide Elongation Factors / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D010802:Phylogeny; D011487:Protein Conformation; D016415:Sequence Alignment
D019843:Archaeal Proteins; D010445:Peptide Elongation Factors
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