OPT domains (Oct1/PTF/Transcription), are transcriptionally active domains rich in PTF, Oct1, TBP, SP1, and RNA pol II (reviewed in Spector, 2001) . These domains are 1.0-1.5 mm in diameter and, in addition to transcription factors, they also contain nascent transcripts (Grande et al., 1997; Pombo et al., 1998) . Each mammalian nucleus contains between one to three OPT domains that appear in G1, usually in close proximity to nucleoli, only to disappear in S phase.

Although the function of these bodies is unkown, a small region on chromosome 6 (band 6p21) and chromosome 7, associate with the domain significantly more than other chromosomes (Pombo et al., 1998) . Therefore OPT domains, like

nucleoli, appear to co-ordinate the association of particular genes on specific chromosomes together in a region where the appropriate transcription and processing factors are concentrated, thereby facilitating the expression of those genes (see the Figure 1; Pombo et al., 1998) .
Fig. 1. Similarities between nucleoli and OPT domains.

A nucleolus contains several (polymerase I) transcription factories; each nucleolus can associate with rDNA genes carried on several chromosomes. The OPT domain may be an analogous region containing transcription factories rich in polymerases II and/or III, and which tends to associate with specific chromosomes. Although OPT domains often lie next to nucleoli, they never overlap them. (Adapted from Fig. 7; Pombo et al., 1998)


REFERENCES

Grande MA, van der Kraan I, de Jong L, van Driel R. Nuclear distribution of transcription factors in relation to sites of transcription and RNA polymerase II. J. Cell Sci. 110 (15):1781-1791.

Pombo A, Cuello P, Schul W, Yoon JB, Roeder RG, Cook PR, 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

Spector, D.L. (2001) Nuclear domains. J. Cell Sci. 114 (16):2891-2893