Ribin, a protein encoded by a message complementary to rRNA, modulates ribosomal transcription and cell proliferation

The control of rRNA transcription, tightly coupled to the cell cycle and gr owth state of the cell, is a key process for understanding the mechanisms t hat drive cell proliferation. Here we describe a novel protein, ribin, foun d in rodents, that binds to the rRNA promoter and stimulates its activity. The protein also interacts with the basal rRNA transcription factor UBF. Th e open reading frame encoding ribin is 96% complementary to a central regio n of the large rRNA. This demonstrates that ribosomal DNA-related sequences in: higher eukaryotes can be expressed as protein-coding messages. Ribin c ontains two predicted nuclear localization sequence elements, and green flu orescent protein-ribin fusion proteins localize in the nucleus. Cell lines overexpressing ribin exhibit enhanced rRNA transcription and faster growth. Furthermore, these cells significantly overcome the suppression of rRNA sy nthesis caused by serum deprivation. On the other hand, the endogenous ribi n level correlates positively with the amount of serum in the medium. The d ata show that ribin is a limiting stimulatory factor for rRNA synthesis in vivo and suggest its involvement in the pathway that adapts ribosomal trans cription and cell proliferation to physiological: changes.