P54nrb is a protein implicated in multiple nuclear processes whose specific 
functions may correlate with its presence at different nuclear locations. Here 
we characterize paraspeckles, a subnuclear domain containing p54nrb and other 
RNA-binding proteins including PSP1, a protein with sequence similarity to 
p54nrb that acts as a marker for paraspeckles. We show that PSP1 interacts in 
vivo with a subset of the total cellular pool of p54nrb. We map the domain 
within PSP1 that is mediating this interaction and show it is required for the 
correct localization of PSP1 to paraspeckles. This interaction is necessary but 
not sufficient for paraspeckle targeting by PSP1, which also requires an RRM 
capable of RNA binding. Blocking the reinitiation of RNA Pol II transcription at 
the end of mitosis with DRB prevents paraspeckle formation, which recommences 
after removal of DRB, indicating that paraspeckle formation is dependent on RNA 
Polymerase II transcription. Thus paraspeckles are the sites where a subset of 
the total cellular pool of p54nrb is targeted in a RNA Polymerase II-dependent 
manner.