Characterization of a cDNA Encoding Ribosomal Protein S16 in Rice

Eukaryotic cytosolic ribosomes are composed of four RNA species and about 80 ribosomal proteins. The assembly of a functional ribosome requires the coordinate expression of genes for a11 constituent ribosomal proteins as well as rRNAs (Amaldi et al., 1989). Molecular dissection of animal ribosomal protein genes revealed that sequence elements embedded in the promoters and the 5‘ end of mature transcripts play critica1 roles in transcriptional and translational control of their expression (Hariharan and Perry, 1990). Relatively little is known about the regulation of plant ribosomal protein gene expression. Over the last few years, an increasing number of plant ribosomal protein genes have been cloned and sequenced (Taylor and Davies, 1994, and refs. therein). Such structural information is an essential early step in exploring the function of the genes. In the course of studying developmental and environmental regulation of the r ice (Oryza sativa L.) 4-coumarate COA ligase gene, we isolated a cDNA clone that was expressed constitutively in various tissues and in different physiological contexts (Table I). A partia1 sequence of the cDNA clone showed high homology with human ribosomal protein S16 (Batra et al., 1991), a component of the 40s subunit of the cytosolic ribosome. Our interest in characterizing this clone further includes the fact that in gene regulation experiments involving RNA quantification, such as RNA gel blotting or run-on assays, an invariant internal control is often required. Although there were observations that the transcript levels of certain ribosomal protein genes vary among different tissues (Larkin et al., 1989), the use of ribosomal protein S26 and L27 genes as the internal controls had been successful (Lebeau et al., 1991; Vincent et al., 1993). We were curious if the rice S16 sequence could possibly serve as an internal standard in our studies on rice genes. Also, although S16 genes have been characterized from human, rat, mouse, and Lupinus polyphyllus (Batra et al., 1991; Warskulat et al., 1991), no information was available from any monocot plant. It would be useful to add the rice S16 data for comparative and evolutionary studies.