Structure and evolution of the Tetrahymena thermophila gene encoding ribosomal protein L21

The single-copy gene encoding ribosomal protein (r-protein) L21 in the macronucleus of the ciliate Tetrahymena thermophila was cloned and characterized. Sequencing of the L21 gene and a corresponding cDNA clone showed the gene to contain three introns, all located in the 3' half of the coding region. Primer extension and nuclease protection analyses revealed five transcription start points (tsp) 56-73 nucleotides upstream from the start codon. The uppermost tsp mapped to the first T in a sequence, TATAA, often found at tsp in T. thermophila. A comparison of amino acid sequences of r-proteins revealed that T. thermophila L21 belongs to a family of r-proteins that have been conserved in eubacteria, archaebacteria and eukaryotes. On the basis of structural and functional considerations, we suggest that the eukaryotic, like the prokaryotic, members of this protein family interact with the 5S RNA complex in ribosomes.