Nucleotide sequence determining the first cleavage site in the processing of XXXX precursor rRNA.

@entity19 @entity1 @entity2036
The first step in the processing of 47S precursor rRNA in @entity19 cells is reproduced in vitro in an S-100 transcription reaction and consists of an endonucleolytic cleavage at residue +650 of the primary transcript followed by rapid degradation of the fragment upstream from residue +650. An analogous processing occurs in @entity1 rRNA. The @entity19 and @entity1 rRNA sequences are approximately equal to 80% conserved for 200 @entity2036 on the 3' side of these processing sites, suggesting that this conserved region may be important in specifying the processing. To test this hypothesis, we constructed a systematic series of deletion mutants approaching the @entity19 rDNA processing region from both the 5' and 3' directions and analyzed the processing of their transcripts in vitro. The 5' boundary of the region required for processing is quite sharp and corresponds to the rRNA cleavage site at the 5' end of the conserved sequence region. The 3' boundary is more complex: The 3' deletions extending to between 250 and 130 @entity2036 beyond the processing site cause about a 50% decrease in the amount of the processed RNA. A 3' deletion that extends to 109 @entity2036 beyond the processing site greatly reduces the processing efficiency. Deletions to or beyond 91 @entity2036 on the 3' side of the processing site virtually eliminate processing. Under altered ionic conditions, transcripts of 3' deletions extending to only 41 @entity2036 beyond the processing site can still direct a low level of accurate processing. These results demonstrate that the @entity19 / @entity1 conserved sequence just on the 3' side of the primary rRNA processing site consists of several domains that direct and/or augment both the initial endonucleolytic cleavage and the closely coupled selective degradation of the upstream fragment that together constitute the primary rRNA processing event.
