Figure 1.

A scheme illustrating epigenetic changes in the alphoid^tetO-HAC caused by tTS expression. (a) The condition when tTS is highly expressed from a retroviral vector in trans. Multiple tTS molecules bind to a great fraction of tetO sequences in the HAC and as previously shown lead to the formation of heterochromatin, spreading of which disrupts centrochromatin. As a result, the HAC kinetochore is inactivated, and the HAC is lost. (b and c) Conditions where a single copy of tTS is integrated into the HAC. The main difference of this system compared with viral expression is that fewer targets (tetO sequences) are bound by the effectors (tTS protein), as less protein is synthesized. Two scenarios are possible. (b) Despite the limited number of tTS molecules binding to tetO sequences in the HAC, active spreading of heterochromation induced by tTS results in the disruption of centrochromatin domains. (c) Binding of the limited number of tTS molecules to alphoid DNA array induces formation of the local chromatin domains that do not compromise activity of the HAC kinetochore but completely blocks transcription of the sequences within these domains. One of these domains is located near the gene-loading site leading to self-silencing of the tTS gene and transgenes nearby.