Figure 1: Alternative path-ways of the pre-rRNA processing in yeast. Left: When nutrients are available and no environmental stress is present, TORC1 and CK2 kinase are active, promoting high level of rDNA transcription, cleavage of the pre-rRNA at the site A2 and maturation of new ribosomes. Right: In the condition with low nutrients or stress, both TORC1 and CK2 are inactive. The transcription of rDNA is reduced and pre-rRNA is cleaved at the site A3 and no new ribosomes are produced.