Identification of a protein factor and the nucleotide sequence required for processing of mouse precursor rRNA

We have studied the protein components and nucleotide sequence involved in the endonucleolytic cleavage at 105 bp upstream of the 5' end of mouse 18S rRNA. By fractionating the mouse S100 extract into four fractions by phosphocellulose chromatography, we separated and characterized the processing factor which participated in in vitro coupled and uncoupled transcription-processing systems. Processing activity was recovered in the flow-through fraction (fraction A), while the transcription initiation was supported by the combination of fractions C and D. This endonucleolytic cleavage activity was heat-labile and resistant to micrococcal nuclease. By constructing deletion fragments, we found that the sequence between 219 bp upstream and 26 bp downstream of the processing site was required for the processing. This sequence does not include the 18S rRNA coding region and can form a stem-loop secondary structure.