EXPRESSION OF CDK7 CAK IN NORMAL AND TUMOR-CELLS OF DIVERSE HISTOGENESIS, CELL-CYCLE POSITION AND DIFFERENTIATION/

The cyclin-dependent kinase 7 (CDK7) represents the 40-kDa catalytic s ubunit of the CDK-activating kinase, the enzyme responsible for activa tory phosphorylation of multiple CDKs controlling GI, S and G(2)/M pha ses of the cell cycle. Here, we surveyed a wide range of normal and tu mour cell types, in both cell culture and biopsy specimens, for abunda nce and subcellular localisation of the CDK7 protein. Immunoblotting a nd immunocytochemical analyses showed that CDK7 was (i) ubiquitously e xpressed in all cell types examined; (ii) exclusively nuclear; (iii) m oderately elevated in tumour cells when compared with their normal cel l counterparts; (iv) invariant throughout the cell cycle of normal lym phocytes, fibroblasts, breast epithelium and several cancer cell lines ; and (v) clearly detectable even in quiescent cells, including highly differentiated cell types in situ. Our data are consistent with the e merging role for CDK7/CAK in multiple biological processes, possibly p roviding a link between cell-cycle control, transcriptional regulation and genomic integrity control. (C) 1996 Wiley-Liss, Inc.