RNA to protein

The Bacterial and Archaeal ribosomes resemble eukaryotic ribosomes in several respects. The prokaryotes lack any compartmentalization of the processes involved in contrast to the many membranous structures the eukaryotes have evolved. Key differences among the prokaryotes and the eukaryotes are listed in Table 1. Prokaryotic ribosomes move around in the cytoplasm freely much as those in the eukaryotic mitochondrion do. The actual mechanism through which mRNA is translated into the polypeptide is very same in all organisms. A detailed description of the process can be found in other books within the Instant Notes series.

                                Table 1.  Differences between eukaryotes and prokaryotes displayed during translation

In Bacteria it has been observed from electron micrographs that transcription and translation are tightly coupled, with translation beginning as soon as a nascent mRNA emerges from the RNA polymerase complex. This is in marked contrast to eukaryotes where mRNA must first pass through the nuclear membrane before translation can occur and in many cases mRNA is targeted towards particular organelles.

In Bacteria, as soon as an RBS emerges from an RNA polymerase during mRNA synthe- sis, a ribosome binds and begins translation. As more mRNA is synthesized the ribosome moves down the codons until there is enough room for another ribosome to bind at the RBS. By the time the RNA polymerase has completed synthesis of mRNA; many ribosomes will be attached, each with a partially completed polypeptide. This multi- ribosome-, mRNA-, and RNA polymerase-containing structure is sometimes called the polysome. The proximity of the RNA polymerase and ribosome plays an important role in the regulation of some genes. In an organism growing quickly on a readily available carbon and energy source the polysome may also be spatially very close to the replisome and the interplay among the two is the source of speculation with regard to regulatory mechanisms.