RNA B Is the Major Nucleolar Trimethylguanosine-Capped Small Nuclear RNA Associated with Fibrillarin and Pre-rRNAs in Trypanosoma brucei

RNA B is one of three abundant trimethylguanosine-capped U small nuclear RNAs (snRNAs) of Trypanosoma brucei which is not strongly identified with other U snRNAs by sequence homology. We show here that RNA B is a highly diverged U3 snRNA homolog likely involved in pre-rRNA processing. Sequence identity between RNA B and U3 snRNAs is limited; only two of four boxes of homology conserved between U3 snRNAs are obvious in RNA B. These are the box A homology, specific for U3 snRNAs, and the box C homology, common to nucleolar snRNAs and required for association with the nucleolar protein, fibrillarin. A 35-kDa T. brucei fibrillarin homolog was identified by using an anti-Physarum fibrillarin monoclonal antibody. RNA B and fibrillarin were localized in nucleolar fractions of the nucleus which contained pre-rRNAs and did not contain nucleoplasmic snRNAs. Fibrillarin and RNA B were precipitated by scleroderma patient serum S4, which reacts with fibrillarins from diverse organisms; RNA B was the only trimethylguanosine-capped RNA precipitated. Furthermore, RNA B sedimented with pre-rRNAs in nondenaturing sucrose gradients, similarly to U3 and other nucleolar snRNAs, suggesting that RNA B is hydrogen bonded to rRNA intermediates and might be involved in their processing.