※ OPT domain introduction

    In some cells, PTF is concentrated in one, or very few, discrete regions (diameter approximately 1.3 micron) that appear during G1 phase and disappear in S phase. Oct1, a transcription factor that interacts with PTF, is also enriched in these domains, RNA polymerase II, TBP and Sp1 are also present. Each domain typically contains 2 or 3 transcription 'factories' where Br-UTP is incorporated into nascent transcripts. This region was named Oct1/PTF/transcription (OPT) domain. The domains may act like nucleoli to bring particular genes on specific chromosomes together to a region where the appropriate transcription and processing factors are concentrated, thereby facilitating the expression of those genes (1). In fact, incomplete DNA synthesis during S phase leads to a DNA damage response and formation of 53BP1-OPT domains in the subsequent G1, which indicates that replication stress induces 53BP1-containing OPT domains in G1 cells (2).

Reference
1. Pombo, A., Cuello, P., Schul, W., Yoon, J. B., Roeder, R. G., Cook, P. R., & Murphy, S. (1998) Regional and temporal specialization in the nucleus: a transcriptionally-active nuclear domain rich in PTF, Oct1 and PIKA antigens associates with specific chromosomes early in the cell cycle. EMBO J, 17(6), 1768-1778. PMID: 9501098
2. Harrigan, J. A., Belotserkovskaya, R., Coates, J., Dimitrova, D. S., Polo, S. E., Bradshaw, C. R., et al. (2011) Replication stress induces 53BP1-containing OPT domains in G1 cells. J Cell Biol, 193(1), 97-108. PMID: 21444690