The yeast nucleolar protein Rrp5p is the only known
trans-acting factor that is essential for the
synthesis of both 18S rRNA and the major, short form of
5.8S (5.8SS) rRNA, which were thought to be
produced in two independent sets of pre-rRNA processing
reactions. To identify domains within Rrp5p required for
either processing pathway, we have analyzed a set of eight
deletion mutants that together cover the entire RRP5
sequence. Surprisingly, only one of the deletions is lethal,
indicating that regions encompassing about 80% of the protein
can be removed individually without disrupting its essential
biological function. Biochemical analysis clearly demonstrated
the presence of two distinct functional domains. Removal
of each of three contiguous segments from the N-terminal
half specifically inhibits the formation of 5.8SS
rRNA, whereas deleting part of the C-terminal region of
the protein only blocks the production of 18S rRNA. The
latter phenotype is also caused by a temperature-sensitive
mutation within the same C-terminal region. The two functional
regions identified by the mutational analysis appear to
be correlated with the structural domains detected by computer
analysis. They can even be physically separated, as demonstrated
by the fact that full Rrp5p activity can be supplied by
two contiguous protein fragments expressed in trans.