Proteins engaged in macromolecular complexes interacting with chromatin move throughout the nucleus by diffusive processes, transiently and repetitively contacting their target sites. Diffusion and interactions play important roles in gene expression by defining the target gene search time. In the work presented in this thesis we monitored the mobility of the Positive Transcription Elongation factor b (P-TEFb) responsible for the phosphorylation of the RNA polymerase II C-terminal domain and showed that its relatively high mobility is regulated by the 7SK small nuclear ribonucleoprotein complex (7SK snRNP). Without the 7SK regulating complex, transient interactions of the P-TEFb cycline T1 component with RNA polymerase II prevent the nuclear distribution of the elongation factor therefore restricting its activity to sites of high polymerase concentration and limiting its recruitment to new sites. This observation demonstrates that the fast mobility of proteins required for the stochastic assembly of nuclear molecular machines is not only a physical property of macromolecules but a biophysical parameter tightly controlled by cellular regulators.
