Transcription by RNA polymerases I and III: a potential link between cell growth, protein synthesis and the retinoblastoma protein.

The rate of protein synthesis is a critical determinant of cellular growth. Abnormal activation of this process is a frequent feature of transformed and tumour cells. Several distinct components of the translation apparatus have been shown to be deregulated in response to malignant transformation. Indeed, overexpression of certain translation factors has been found to predispose cells to transformation or even initiate it. The latest twist to this story comes from the discovery that the retinoblastoma protein RB plays a major role in restricting the production of tRNA and rRNA. RB is an important tumour suppressor. Its ability to limit the synthesis of these principle determinants of biosynthetic capacity could provide a mechanism for restraining cell growth. The loss of this control may constitute a significant step towards tumour progression.