KEY TERMS:
- A polyribosome (Polysome) is an mRNA that is simultaneously being translated by several ribosomes.
- A nascent protein has not yet completed its synthesis; the polypeptide chain is still attached to the ribosome via a tRNA.
- An mRNA is simultaneously translated by several ribosomes. Each ribosome is at a different stage of progression along the mRNA.
When active ribosomes are isolated in the form of the
fraction associated with newly synthesized proteins, they are found in the form
of a complex consisting of an mRNA associated with several ribosomes. This is
the polyribosome or polysome. The 30S subunit of each ribosome is associated
with the mRNA, and the 50S subunit carries the newly synthesized protein. The
tRNA spans both subunits.
Each ribosome in the polysome independently synthesizes a
single polypeptide during its traverse of the messenger sequence. Essentially
the mRNA is pulled through the ribosome, and each triplet nucleotide is
translated into an amino acid. So the mRNA has a series of ribosomes that carry
increasing lengths of the protein product, moving from the 5
Roughly the most recent 30-35 amino acids added to a growing
polypeptide chain are protected from the environment by the structure of the
ribosome. Probably all of the preceding part of the polypeptide protrudes and is
free to start folding into its proper conformation. So proteins can display
parts of the mature conformation even before synthesis has been completed.
A classic characterization of polysomes is shown in the
electron micrograph of Figure 5.10. Globin protein is
synthesized by a set of 5 ribosomes attached to each mRNA (pentasomes). The
ribosomes appear as squashed spherical objects of ~7 nm (70 Å) in diameter, connected by a thread of mRNA. The
ribosomes are located at various positions along the messenger. Those at one end
have just started protein synthesis; those at the other end are about to
complete production of a polypeptide chain (Slayter et al.,
1963).
The size of the polysome depends on several variables. In
bacteria, it is very large, with tens of ribosomes simultaneously engaged in
translation. Partly the size is due to the length of the mRNA (which usually
codes for several proteins); partly it is due to the high efficiency with which
the ribosomes attach to the mRNA.
Polysomes in the cytoplasm of a eukaryotic cell are likely
to be smaller than those in bacteria; again, their size is a function both of
the length of the mRNA (usually representing only a single protein in
eukaryotes) and of the characteristic frequency with which ribosomes attach. An
average eukaryotic mRNA probably has ~8 ribosomes attached at any one
time.
Figure 5.11 illustrates the life
cycle of the ribosome. Ribosomes are drawn from a pool (actually the pool
consists of ribosomal subunits), used to translate an mRNA, and then return to
the pool for further cycles. The number of ribosomes on each mRNA molecule
synthesizing a particular protein is not precisely determined, in either
bacteria or eukaryotes, but is a matter of statistical fluctuation, determined
by the variables of mRNA size and efficiency.
An overall view of the attention devoted to protein
synthesis in the intact bacterium is given in Figure
5.12. The 20,000 or so ribosomes account for a quarter of the cell mass.
There are >3000 copies of each tRNA, and
altogether, the tRNA molecules outnumber the ribosomes by almost tenfold; most
of them are present as aminoacyl-tRNAs, that is, ready to be used at once in
protein synthesis. Because of their instability, it is difficult to calculate
the number of mRNA molecules, but a reasonable guess would be ~1500, in varying
states of synthesis and decomposition. There are ~600 different types of mRNA in
a bacterium. This suggests that there are usually only 2-3 copies of each mRNA
per bacterium. On average, each probably codes for ~3 proteins. If there are
1850 different soluble proteins, there must on average be >1000 copies of each protein in a bacterium.