Endonucleolytic cleavage within Internal Transcribed Spacer 1 (ITS1) upstream of the 5.8S rRNA derived from an originally tricistronic rRNA transcript that contained the Small Subunit (SSU) rRNA, the 5.8S rRNA, and the Large Subunit (LSU) rRNA in that order from 5' to 3' along the primary transcript. In S. cerevisiae, this endonucleolytic cleavage within ITS1 initiates the maturation of the LSU and the 5.8S rRNAs.
endonucleolytic cleavage at A3