The roles of S1 RNA-binding domains in Rrp5's interactions with pre-rRNA.

RNA-binding proteins mediate the function of all RNAs. Since few distinct RNA-binding domains (RBDs) exist, with most RBDs contacting only a few nucleotides, RNA-binding proteins often combine multiple RNA-binding motifs to achieve a higher affinity and selectivity for their targets. Rrp5, a ribosome assembly factor essential for both 40S and 60S ribosome maturation, is an extreme example as it contains 12 tandem S1 RNA-binding domains. In this study, we use a combination of RNA binding and DMS probing experiments to probe interactions of Rrp5 with pre-rRNA mimics. Our data localize Rrp5's binding site to three distinct regions within internal transcribed spacer 1 (ITS1), the sequence between 18S and 5.8S rRNAs. One of these regions is directly adjacent to a recently uncovered helical structure, which prevents premature cleavage at the 3'-end of 18S rRNA. This finding, together with previous results, suggests a role for Rrp5 in regulating the above-mentioned helical element. Furthermore, we have produced two truncated forms of the protein, Rrp5N and Rrp5C, which together encompass the entire protein and fully restore growth. Quantitative analysis of the RNA affinity of these Rrp5 fragments indicates that the first nine S1 motifs contribute much of Rrp5's RNA affinity, while the last three domains alone provide its specificity for the pre-rRNA. This surprising division of labor is unique, as it suggests that S1 domains can bind RNA both specifically as well as nonspecifically with high affinity; this has important implications for the molecular details of the Rrp5•pre-rRNA complex.