Archaeal ribosomal protein L7 is a functional homolog of the eukaryotic 15.5kD/Snu13p snoRNP core protein

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Abstract
Recent investigations have identified homologs of eukaryotic box C/D small nucleolar RNAs (snoRNAs) in Archaea termed sRNAs. Archaeal homologs of the box C/D snoRNP core proteins fibrillarin and Nop56/58 have also been identified but a homolog for the eukaryotic 15.5kD snoRNP protein has not been described. Our sequence analysis of archaeal genomes reveals that the highly conserved ribosomal protein L7 exhibits extensive homology with the eukaryotic 15.5kD protein. Protein binding studies demonstrate that recombinant Methanoccocus jannaschii L7 protein binds the box C/D snoRNA core motif with the same specificity and affinity as the eukaryotic 15.5kD protein. Identical to the eukaryotic 15.5kD core protein, archaeal L7 requires a correctly folded box C/D core motif and intact boxes C and D. Mutational analysis demonstrates that critical features of the box C/D core motif essential for 15.5kD binding are also required for L7 interaction. These include stem I which juxtaposes boxes C and D, as well as the sheared G:A pairs and protruded pyrimidine nucleotide of the asymmetric bulge region. The demonstrated presence of L7Ae in the Haloarcula marismortui 50S ribosomal subunit, taken with our demonstration of the ability of L7 to bind to the box C/D snoRNA core motif, indicates that this protein serves a dual role in Archaea. L7 functioning as both an sRNP core protein and a ribosomal protein could potentially regulate and coordinate sRNP assembly with ribosome biogenesis.