The nucleolus is an organelle found in the nucleus of eukaryotic cells. It forms around
specific chromosomal regions in the nucleus and is made up of proteins and RNA. Its primary
function is to transcribe and modify ribosomal RNA and integrate ribosomal proteins to form
immature ribosomes. It is also part of eukaryotic cells' stress response, acting as a stress
sensor and is able to regulate rRNA synthesis based on the cellular environment.
The nucleolus occupies up to about 25% of the volume of the cell nucleus. Malfunction
of nucleoli can be the cause of several human diseases.
History Before 1964, the nucleolus was recognized
as a morphological element within the nucleus, but little was known of its function. During
that year, a study of nucleoli by John Gurdon and Donald Brown in the African clawed frog
Xenopus laevis, generated increasing interest in the function and detailed structure of
the nucleolus. They found that 25% of the frog eggs had no nucleolus and that such eggs
were not capable of life. Half of the eggs had one nucleolus and 25% had two. They concluded
that the nucleolus had a function necessary for life. In 1966 Max L. Birnstiel and Hugh
Wallace showed via hybridization experiments that genes within nucleoli code for ribosomal
RNA. Structure
Three major components of the nucleolus are recognized: the fibrillar center, the dense
fibrillar component, and granular component. The DFC consists of newly transcribed rRNA
bound to ribosomal proteins, while the GC contains RNA bound to ribosomal proteins that
are being assembled into immature ribosomes. However, it has been proposed that this particular
organization is only observed in higher eukaryotes and that it evolved from a bipartite organization
with the transition from anamniotes to amniotes. Reflecting the substantial increase in the
DNA intergenic region, an original fibrillar component would have separated into the FC
and the DFC. Another structure identified within many nucleoli
is a clear area in the center of the structure referred to as a nucleolar vacuole. Nucleoli
of various plant species have been shown to have very high concentrations of iron in contrast
to human and animal cell nucleoli. The nucleolus ultrastructure can be visualized
through an electron microscope, while the organization and dynamics can be studied through
fluorescent protein tagging and fluorescent recovery after photobleaching. Antibodies
against the PAF49 protein can also be used as a marker for the nucleolus in immunofluorescence
experiments. Function and ribosome assembly
Nucleoli are formed around specific genetic loci called nucleolar organizing regions,
first described by Barbara McClintock. Because of this non-random organization, the nucleolus
is defined as a "genetically determined element." An NOR is composed of tandem repeats of rDNA,
which can be found on several different chromosomes. The human genome, for example, contains more
than 200 clustered copies of rDNA on five different chromosomes. In a typical eukaryote
rDNA consists of a promoter, internal and external transcribed spacers, rRNA coding
sequences and an intergenic spacer. In ribosome biogenesis, two of the three eukaryotic
RNA polymerases are required, and these function in a coordinated manner. In an initial stage,
the rRNA genes are transcribed as a single unit within the nucleolus by RNA polymerase
I. In order for this transcription to occur, several pol I-associated factors and DNA-specific
trans-acting factors are required. In yeast, the most important are: UAF, TBP, and CF,
which bind promoter elements and form the preinitiation complex, which is in turn recognized
by RNA pol. In humans, a similar PIC is assembled with SLI, the promoter selectivity factor,
IFs and UBF. RNA polymerase I transcribes most rRNA transcripts but the 5S rRNA subunit
is transcribed by RNA polymerase III. Transcription of rRNA yields a long precursor
molecule which still contains the ITS and ETS. Further processing is needed to generate
the 18S RNA, 5.8S and 28S RNA molecules. In eukaryotes, the RNA-modifying enzymes are
brought to their respective recognition sites by interaction with guide RNAs, which bind
these specific sequences. These guide RNAs belong to the class of small nucleolar RNAs
which are complexed with proteins and exist as small-nucleolar-ribonucleoproteins. Once
the rRNA subunits are processed, they are ready to be assembled into larger ribosomal
subunits. However, an additional rRNA molecule, the 5S rRNA, is also necessary. In yeast,
the 5S rDNA sequence is localized in the intergenic spacer and is transcribed in the nucleolus
by RNA pol. In higher eukaryotes and plants, the situation
is more complex, for the 5S DNA sequence lies outside the NOR and is transcribed by RNA
pol III in the nucleoplasm, after which it finds its way into the nucleolus to participate
in the ribosome assembly. This assembly not only involves the rRNA, but ribosomal proteins
as well. The genes encoding these r-proteins are transcribed by pol II in the nucleoplasm
by a "conventional" pathway of protein synthesis. The mature r-proteins are then "imported"
back into the nucleus and finally the nucleolus. Association and maturation of rRNA and r-proteins
result in the formation of the 40S and 60S subunits of the complete ribosome. These are
exported through the nuclear pore complexes to the cytoplasm, where they remain free or
become associated with the endoplasmic reticulum, forming rough endoplasmic reticulum.
A continuous chain between the nucleoplasm and the inner parts of the nucleolus exists
through a network of nucleolar channels. In this way, macromolecules with a molecular
weight up to 2000 kDa are easily distributed throughout the nucleolus.
Sequestration of proteins In addition to its role in ribosomal biogenesis,
the nucleolus is known to capture and immobilize proteins, a process known as nucleolar detention.
Proteins that are detained in the nucleolus are unable to diffuse and to interact with
their binding partners. Targets of this post-translational regulatory mechanism include VHL, PML, MDM2,
POLD1, RelA, HAND1 and hTERT, among many others. It is now known that long noncoding RNAs originating
from intergenic regions of the nucleolus are responsible for this phenomenon.
References
External links Cooper, Geoffrey M.. "The Nucleolus". The
Cell: A Molecular Approach. Sunderland MA: Sinauer Associates. ISBN 0-87893-106-6.
Nucleolus under electron microscope II at uni-mainz.de
Nuclear Protein Database – search under compartment
Cell Nucleolus at the US National Library of Medicine Medical Subject Headings
Histology image: 20104loa - Histology Learning System at Boston University