Explanatory sketch of the patterns for genes,
transcripts, and splicing factors. Blue helices, genes; red and gray
segments, RNA; black, gray, and orange dots, splicing factors; orange
area, speckle; orange arrows, recruitment of splicing factors;
concentrated dots in the center with the overall ellipsoidal shape,
IGC; dots and the gray segments outside of the IGC, PFs. Blue, red, and
orange correspond to fluorescence images; EM is in grayscale. (I)
Transcription is moderate (or even high), and co-transcriptional
splicing is at a low or moderate level. The recruitment of splicing
factors is relatively low; much of the unprocessed pre-mRNA is
trafficking to IGC. Here, the RNA trafficking clearly prevails over the
recruitment of splicing factors, and visualized RNA tracks are
associated with the splicing factor reservoirs as observed in Namalwa
cells. (II) Example similar to the previous case, but
co-transcriptional splicing prevails. There is an elevated recruitment
of splicing factors, and (most of) the visualized RNA (RNA tracks)
becomes part of the speckle. The corresponding gene becomes associated
with the speckle. (III) Example of the local site of high
transcriptional activity (depicted here from the clustered genes) and
both co- and post-transcriptional splicing. The splicing factors are
highly recruited, and both the RNA (as observed in Raji cells) and the
genes are extensively engulfed in the speckle. This example is similar
to II. (IV) Example of a highly transcribed gene, high recruitment of
splicing factors, and co-transcriptional splicing only. Both the RNA
(as a spot) and the gene are associated with the speckle. This pattern
has been observed rarely in the present study. (V) Example of an
endogenous gene expressed at a low level. Transcription is low, as is
the recruitment of splicing factors. Visualized RNA appears as a spot
at the site of transcription (because of relatively elevated local RNA
accumulation), and any directed movement of released RNA is below the
level of detection. However, this situation also corresponds to the
expressed genes after resumption of RNA synthesis, as seen after DRB
treatment of Namalwa cells in this study.