Abstract
In eukaryotes, formation of short duplexes between the U3 small nucleolar RNA (snoRNA) and the precursor rRNA (pre-rRNA) at multiple sites is a prerequisite for three endonucleolytic cleavages that initiate small subunit biogenesis by releasing the 18S rRNA precursor from the pre-rRNA. The most likely role of these RNA duplexes is to guide the U3 snoRNA and its associated proteins, designated the small subunit processome, to the target cleavage sites on the pre-rRNA. Studies by others in Saccharomyces cerevisiae have identified the proteins Mpp10p, Imp3p, and Imp4p as candidates to mediate U3-pre-rRNA interactions. We report here that Imp3p and Imp4p appear to stabilize an otherwise unstable duplex between the U3 snoRNA hinge region and complementary bases in the external transcribed spacer of the pre-rRNA. In addition, Imp4p, but not Imp3p, seems to rearrange the U3 box A stem structure to expose the site that base-pairs with the 5' end of the 18S rRNA, thereby mediating duplex formation at a second site. By mediating formation of both essential U3-pre-rRNA duplexes, Imp3p and Imp4p may help the small subunit processome to dock onto the pre-rRNA, an event indispensable for ribosome biogenesis and hence for cell growth.