An Archaebacterial elF‐1A: new grist for the mill

The archaebacteria (or Archaea) were shown, by molecular phylogenetecists, to be the closest prokaryotic relatives of the nuclear component of eukaryotic cells (Iwabe et al., 1989, Proc Nat/Acad Sci USA 86: 9355-9359). Many aspects of their molecular biology have also been found to support this conclusion, by demonstrating close similarity to the corresponding eukaryotic systems. There are several good examples of this, including the proteasome, small subunit ribosomal RNA processing, and, most convincingly, the transcription complex (PQhler et al., 1994, Syst App Microbiol 16: 734-741; Duravic and Dennis, 1994, Mol Microbio113: 229-242; reviewed in Keeling et al., 1994, Curr Op Genet Dev 4: 816-822). In other characteristics, the archaebacteria bear a closer resemblance to their eubacterial cousins: genome and gene structure are shared, as well as many features of the translational process. It is well known, for instance, that archaebacterial mRNAs are often polycistronic, uncapped and do not include a long poly-A tail as do eukaryotic transcripts. Moreover, ribosome binding to mRNA is thought to be effected by a Shine-Dalgarno-like interaction between the 16S ribosomal RNA and mRNA (Brown and Daniels, 1989, CRC Crit Rev Microbio116: 287-335) all features which suggest that archaebacterial translational initiation is possibly similar to that in eubacteria and unlike that in eukaryotes. However, what little direct evidence there is bearing on the nature of the archaebacterial translation-initiation complex is somewhat contradictory. The single putative initiation factor that has been identified in the archaebacteria is actually homologous to the eukaryotic initiation factor elF-5A (Bartig et aL, 1992, Eur J Biochem 204: 751758), for which no eubacterial equivalent is known. Moreover, the role of elF-5A in eukaryotic translation initiation has recently been seriously questioned (Kang and Hershey, 1994, J Biol Chem 269" 3934-3940), and no functional analysis has been performed on the archaebacterial homologue. Nevertheless, the possibility that archaebacterial translation involves both eukaryotic and eubacterial elements is a tempting speculation, and has prompted a search for other, as yet unidentified, archaebacterial translation-initiation proteins. Using the BLASTP search strategy (Altschul et al., 1990, J Mol Biol 215: 403-410) we compared a representative sequence of each eubacterial and eukaryotic initiation factor, for which a sequence is available, to the existing sequence database. From this search, two archaebacterial reading frames were unambiguously identified: the aforementioned Sulfolobus acidocaldarious elF-5A homoIogue, and a previously unidentified open reading frame (ORF), ORF125 (Fig. 1), located downstream of the Thermoplasma acidophi/um RNA polymerase operon (Klenk et al., 1992, Nucl Acids Res 20: 5226). The inferred polypeptide sequence of ORF125 is approximately 30% identical and 50% similar over the aligned region to elF-1A (also called elF-4C), which is a highly conserved initiation factor recently sequenced from animals, plants (Dever et al., 1994, J Biol Chem 269: 3212-3218), and fungi (GenBank accession number Ul1585). The physical nature of the Thermoplasma elF-1A is somewhat different to that of its eukaryotic counterparts, which have a dipole nature characterized by an acidic C-terminal tail, and a highly basic N-terminus. The archaebacterial sequence has neither of these features, and it is actually missing the 22-31 amino acids corresponding to the C-terminal