AN UNUSUAL FIBRILLARIN GENE AND PROTEIN - STRUCTURE AND FUNCTIONAL IMPLICATIONS

The diploid germinal nucleus of the ciliated protozoan Tetrahymena the rmophila is unusual among eukaryotes in that it encodes a single copy of the gene for rRNA allowing identification of cis-acting mutations i n rDNA affecting rRNA structure, function, and processing. The general ly conserved nucleolar protein fibrillarin has been characterized from a number of systems and is involved in pre-rRNA processing. We have d emonstrated that Tetrahymena has fibrillarin and have analyzed the cDN A and the genomic DNA encoding this protein. The derived amino acid se quence of the N-terminal region of Tetrahymena fibrillarin shows littl e similarity with the generally highly conserved glycine / arginine-ri ch N-terminal domain of other eukaryotic fibrillarins. The remainder o f the amino acid sequence of the molecule is more conserved. Polyclona l antibodies generated against the full-length Tetrahymena fibrillarin expressed in bacteria recognize a protein of M-r approximately 32,000 in whole-cell or nucleolar preparations. Immunocytochemistry localize s fibrillarin to nucleoli in the somatic macronuclei of vegetative cel ls. Transformation experiments demonstrate that fibrillarin is an esse ntial protein in Tetrahymena. The Tetrahymena fibrillarin is expressed but does not complement a NOP1 null mutation when transformed into th e yeast Saccharomyces cerevisiae, indicating less functional conservat ion among fibrillarins than previously suggested.