Fibrillarin [1] is a component of a nucleolar small nuclear ribonucleoprotein (SnRNP) particle thought to participate in the first step of the processing of pre-rRNA. In mammals, fibrillarin is associated with the U3, U8 and U13 small nuclear RNAs [2].

Fibrillarin is an extremely well conserved protein of about 320 amino acid residues. Structurally it consists of three different domains:

• An N-terminal domain of about 80 amino acids which is very rich in glycine and contains a number of dimethylated arginine residues (DMA).
• A central domain of about 90 residues which resembles that of RNA-binding proteins and contains an octameric sequence similar to the RNP-2 consensus found in such proteins.
• A C-terminal α-helical domain.

A protein evolutionary related to fibrillarin has been found [3] in archaebacteria. This protein (gene flpA) is involved in pre-rRNA processing. It lacks the Gly/Arg-rich N-terminal domain.

As a signature pattern, we selected a region that start with and encompass the RNP-2 like octapeptide sequence.