Nucleotide sequence of a rice cDNA encoding a homolog of the eukaryotic ribosomal protein S8.

Ribosomes of eukaryotes as well as prokaryotes are highly ordered ribonucleoprotein particles that serve as the factory of protein synthesis and are built of two subunits, smaller (40s) and larger (603, composed of 70 to 80 proteins and four different rRNA molecules (Wool, 1986). Expression of genes coding for ribosomal proteins, synthesis of these proteins, and their assembly to form functional ribosome molecules are coordinately regulated, but most details of these regulation systems are still unclear. Rat ribosomal protein S8 is identified as a component of the 40s subunit (Chan et al., 1987, and GenBank X06423) and the eukaryotic S8 homolog has already been cloned from some species such as human (GenBank X67247), yeast (GenBank Z26879), and mouse (GenBank X73829). However, no counterpart is known in the prokaryotic 30s subunit corresponding to the eukaryotic 40s subunit of eubacteria, archeabacteria, or organella in eukaryotic cells. We have carried out the random sequencing of a cDNA clone derived from the salt-stressed callus cDNA library of the graminaceous monocot rice (Oryza sativa L.) (Umeda et al., 1994). By comparing partial sequences of cDNA clones to the GenBank data base, we found one clone (SSK7) that is similar to the human ribosomal protein S8 gene. Although a short, partial sequence of the S8 homolog isolated from Zea mays has been described in the EMBL data base (M95064), the entire cDNA sequence of the S8 gene has not been presented in plants. Thus, we were prompted to determine the complete nucleotide sequence of SSK7 (Table I). The cDNA is 849 bp in length, and the deduced open reading frame encodes a 25-kD protein of 220 amino acid residues. The nucleotide sequence shows 63% identity with that of mouse, 62% with rat, 49% with human, and 45% with yeast. As reported for most ribosomal proteins, rice S8 protein has a high pI of 11.8, as do other eukaryotic S8 proteins, because of the high content of basic amino acid residues such as Arg (11%) and Lys (11%). The deduced amino acid sequence of rice S8 protein shows 59% identity with those of human, rat, and mouse,