The gene of the ribosomal protein S8 from Thermus thermophilus VK1 has been isolated from a genomic library by hybridization of an oligonucleotide coding for the N-terminal amino acid sequence of the protein, amplified by PCR and sequenced. Nucleotide sequence reveals an open reading frame coding for a protein of 138 amino acid residues (Mr 15 839). The codon usage shows that 94% of the codons possess G or C in the third position, and agrees with the preferential usage of codons of high G+C content in the bacteria of the genus Thermus. The amino acid sequence of the protein shows 48% identity with the protein from Escherichia coli. Ribosomal protein S8 from 7: thermophilus has been expressed in E. coli under the control of the T7 promoter and purified to homogeneity by heat treatment of the extract followed by cation-exchange chromatography. Conditions were defined in which 7: thermophilus protein S8 binds specifically an homologous 16s rRNA fragment containing the putative S8 binding site with an apparent association constant of 5x10' M-'. The overexpressed protein binds the rRNA with the same affinity as that extracted from 7: thermophilus, indicating that the thermophilic protein is correctly folded in E. coli. The specificity of this binding is dependent on the ionic strength. The protein S8 from 7: thermophilus recognizes the E. coli rRNA binding site as efficiently as the S8 protein from E. coli. This result agrees with sequence comparisons of the S8 binding site on the small subunit rRNA from E. coli and from 7: thermophilus, showing strong similarities in the regions involved in the interaction. It suggests that the structural features responsible for the recognition are conserved in the mesophilic and thermophilic eubacteria, despite structural pecularities in the thermophilic partners conferring thermostability.Although crystals of ribosomes as well as of ribosomal particles diffracting at correct resolution have been obtained (see e.g. Yonath et al., 1990;Yusupov et al., 1991), determination at atomic level of the structure of such multimolecular ribonucleoprotein complexes is obviously a long-term pro-

The gene for the ribosomal protein L22 from Thermus thermophilus has been sequenced and overexpressed in Escherichia coil A multiple sequence alignment was carried out for all proteins of the L22 family reported so far. The recombinant protein was purified and crystallized. The crystals belong to the space group P212~2 . with cell parameters ofa = 32.6 A, b = 66.0 A, c = 67.8 A.

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Crystals have been obtained for recombinant ribosomal protein S8 from Thermus thermophilus produced by Escherichia coli. The protein crystals have been grown in 40 mM potassium phosphate buffer (pH 6.0) in hanging drops equilibrated against saturated ammonium sulfate (unbuffered) with 2-methyl-2,4-pentandiol (v/v). The crystals belong to the space group P4(1(3)) 2(1)2 with cell parameters a = b = 67.65 A, c = 171.12 A. They diffract x-rays to 2.9 A resolution.