Molecular cloning and targeting of a fibrillarin homolog from Arabidopsis

Fibrillarin is a nucleolar protein known to be involved in the processing o f ribosomal RNA precursors. We isolated AtFbr1, a cDNA encoding a homolog o f fibrillarin in Arabidopsis. The cDNA is 1.2 kb in size and encodes a poly peptide of 310 amino acid residues with a molecular mass of 33 kD. AtFbr1 i s expressed at high levels in the flower and root tissue and at a slightly lower level in leaf tissue, whereas it was nearly undetectable in siliques. Expression of AtFbr1 was compared with that of the FLP (fibrillarin-like p rotein) gene identified by the Arabidopsis genome project. Abscisic acid tr eatment resulted in the down-regulation of the expression of both AtFbr1 an d FLP genes in seedlings, although the degree of suppression was higher for FLP than for AtFbr1. In addition, the expression level of FLP decreased wi th the age of the seedlings, whereas AtFbr1 did not exhibit any detectable change. The subcellular localization of AtFbr1 was studied with an in vivo targeting approach using a fusion protein, and was found to be correctly ta rgeted to the nucleolus in protoplasts when expressed as a green fluorescen t fusion protein (GFP). Deletion experiments showed that the N-terminal gly cine- and arginine-rich region is necessary and sufficient to target AtFbr1 to the nucleolus.