Figure 6–37 Transport of a large mRNA molecule through the nuclear pore complex. (A) The maturation of an mRNA molecule as it is synthesized by RNA polymerase and packaged by a variety of nuclear proteins. This drawing of an unusually large and abundant insect RNA, called the Balbiani Ring mRNA, is based on electron microscope micrographs such as that shown in (B). (A, adapted from B. Daneholt, Cell 88:585–588, 1997. With permission from Elsevier; B, from B.J. Stevens and H. Swift, J. Cell Biol. 31:55–77, 1966. With permission from The Rockefeller University Press.) be fluorescently labeled and observed individually. A typical RNA molecule is released from its site of transcription and spends several minutes diffusing to a nuclear pore complex. During this time it is likely that RNA processing events continue and that the RNA sheds previously bound proteins and acquires new ones. Once it arrives at the entrance to the pore, the “export-ready” mRNA hovers for several seconds, during which time the completion of processing may occur, and then is transported through the pore very rapidly, in tens of milliseconds. Some mRNA–protein complexes are very large, and how they move through the nuclear pore complexes so rapidly remains a mystery.
Some of the proteins deposited on the mRNA while it is still in the nucleus can affect the fate of the RNA after it is transported to the cytosol. Thus, the stability of an mRNA in the cytosol, the efficiency with which it is translated into protein, and its ultimate destination in the cytosol can all be determined by proteins acquired in the nucleus that remain bound to the RNA after it leaves the nucleus.
But before discussing what happens to mRNAs in the cytosol, we briefly consider how the synthesis and processing of some noncoding RNA molecules occurs. There are many types of noncoding RNAs produced by cells (see Table 6–1, p. 305), but here we focus on the rRNAs, which are critically important for the translation of mRNAs into protein.
Noncoding RNAs Are Also Synthesized and Processed in the Nucleus
Only a few percent of the dry weight of a mammalian cell is RNA; of that, only about 3–5% is mRNA. The bulk of the RNA in cells performs structural and catalytic functions (see Table 6–1). The most abundant RNAs in cells are the ribosomal RNAs (rRNAs), constituting approximately 80% of the RNA in rapidly dividing cells. As discussed later in this chapter, these RNAs form the core of the ribosome. Unlike bacteria—in which a single RNA polymerase synthesizes all RNAs in the cell—eukaryotes have a separate, specialized polymerase, RNA polymerase I, that is dedicated to producing rRNAs. RNA polymerase I is similar structurally to the RNA polymerase II discussed previously; however, the absence of a C-terminal tail in polymerase I helps to explain why its transcripts are neither capped nor polyadenylated.
Because multiple rounds of translation of each mRNA molecule can provide an enormous amplification in the production of protein molecules, many of the proteins that are very abundant in a cell can be synthesized from genes that are present in a single copy per haploid genome (see Figure 6–3). In contrast, the RNA components of the ribosome are final gene products, and a growing mammalian cell must synthesize approximately 10 million copies of each type of ribosomal RNA in each cell generation to construct its 10 million ribosomes. The cell can produce adequate quantities of ribosomal RNAs only because it contains multiple copies of the rRNA genes that code for ribosomal RNAs (rRNAs). Even
E. coli needs seven copies of its rRNA genes to meet the cell’s need for ribosomes. Human cells contain about 200 rRNA gene copies per haploid genome, spread