Ribosomes can read an mRNA sequence and translate it into protein, defining them as protein factories of a cell. These essential factories are very similar in all orgaisms. Analysis of rRNAs and ribosomal proteins from a variety of organisms, clearly displays that all ribosomes have evolved from a common ancestral ribonucleoprotein (RNP) particle (Wool et al., 1996; Gerbi et al., 1996). With the power of techniques such as X-ray crystallography and cryoelectron microscopy, the ribosomes structure is being mapped out to the finest detials, which will help to elucidate its mechanism in initiation (Dube et al., 1998; Verschoor et al., 1996). The mammalian 80S ribosome is compromised of a 40S and 60S subunits. The 40S subunit is a complex of a 18S rRNA and 33 unique proteins, while the 60S subunit, consists of a 5S, 5.8S and 28S rRNA and 49 unique proteins.
However, even though eukaryotic ribosomes are composed of far more proteins than their prokaryotic counterparts, both structures are very homologous within their working cavities. The site of tRNA binding, the peptidytransferase locations and the site of mRNA decoding share a great degree of similarity (Wilson and Noller, 1998). The 30S subunit of prokaryotes catalyzes the passing of mRNA within ribosomes by clamping incoming mRNA near the decoding area and passing of mRNA within ribosomes by clamping incoming mRNA near the decoding area and passing mRNA through two holes formed by non-covalent interactions (mRNA channel), sites which can be mapped within the 40S subunit of eukaryotes (Dube et al., 1998). These structural similarities between eukaryotic and prokaryotic ribosomes imply that there may be a conserved universal mechanism of translation.
Even though the ribosomes are the protein mechinery of a cell, their work is unattainable without the help from translation factors. Translation initiation factors promote the formation and initiation of the translation machinery.