Figure 6–46 Visualization of some prominent nuclear bodies. The protein fibrillarin (red), a component of several snoRNPs, is present at both nucleoli and Cajal bodies; the latter are indicated by the arrows. The Cajal bodies (but not the nucleoli) are also highlighted by staining one of their main components, the protein coilin; the superposition of the snoRNP and coilin stains appears pink. Interchromatin granule clusters (green) have been revealed by using antibodies against a protein involved in pre-mRNA splicing. DNA is stained blue by the dye DAPI. (From J.R. Swedlow and A.I. Lamond, Gen. Biol. 2:1–7, 2001. With permission from BioMed Central. Micrograph courtesy of Judith Sleeman.) disrupting a particular type of nuclear body often has little effect on cell viability. It seems that the main function of these aggregates is to bring components together at high concentration in order to speed up their assembly. For example, it is estimated that assembly of the U4/U6 snRNP (see Figure 6–28) occurs ten times more rapidly in Cajal bodies than would be the case if the same number of components were dispersed throughout the nucleus. Consequently, Cajal bodies appear dispensible in many types of cells but are absolutely required in situations where cells must proliferate rapidly, such as in early vertebrate development. Here, protein synthesis (which depends on RNA splicing) must be especially rapid, and delays can be lethal.
Given the prominence of nuclear bodies in RNA processing, it might be expected that pre-mRNA splicing would occur in a particular location in the nucleus, as it requires numerous RNA and protein components. However, as we have seen, the assembly of splicing components on pre-mRNA is co-transcriptional; thus, splicing must occur at many locations along chromosomes. Although a typical mammalian cell may be expressing on the order of 15,000 genes, transcription and RNA splicing takes place in only several thousand sites in the nucleus. These sites are highly dynamic and probably result from the association of transcription and splicing components to create small factories, the name given to specific aggregates containing a high local concentration of selected components that create biochemical assembly lines (Figure 6–47). Interchromatin tail of RNA polymerase