Stoffler and Wittmann's Model of Ribosome Structure

Stoffler and Wittmann's Model of Ribosome Structure

	Stoffler and Wittmann's Model of Ribosome Structure - 30S subunit The 30S subunit has been variously described as ellipsoid, oblate ellipsoid, triaxial ellipsoid, disc shaped and obtuse angled scalene triangle by various workers. According to Stoffler and Wittman's model the 30S subunit has an elongated, slightly bent prolate shape. It is a bipartite, structure. A transvers hollow or cleft divides the 30S subunit into two parts, a smaller 'head' and a larger 'body', giving it the appearance of a telephone receiver or embryo. The two lobes protrude unequally and to different extents.
The body has elongated extensions on both sides. In rear views the dorsal surface either appears plain or shows a shallow dent. 50S subunit Electron microscopy observations of E.coli 50S subunits reveal a number of shapes; rounded, kidney shaped, circular profiles with a nose and maple leaf structures.
These different shapes have been interpreted as being the same 50S structure seen in different views. (frontal maple leaf; lateral kidney shaped; rear view rounded). In a frontal view the 50S subunit appears bilaterally symmetrical, and shows three protuberances arising from a rounded base (maple leaf structure). Of these the central protuberance is the most prominent. The 50S sub unit has been compared to an armchair, with the rounded base forming a vaulted seat, the central protuberance the back, and the lateral protuberances the arms. 70S ribosome The 308 and the 50S subunits are associated to form the 70S ribosome. The frontal face of the 30S subunit with its hollow faces the vaulted seat of the 50S subunit. The long axis of the 30S subunit is oriented transversely to the central protuberance of the 50S subunit.
A tunnel is formed between the hollow of the small subunit and the vaulted seat of the large subunit. There are four areas of contact between the two subunits. In area A the frontal region of the head of the 30S subunit is in contact with the central and right lateral protuberances of the 50S subunit. Area B is the region of contact between the right temporal side of the bead of the 30S subunit and the right side of the central protuberance. In area C the triangular anterior region of the 30S body contacts the seat region of the 50S subunit. In area D the right lobe that protrudes from the body of the 30S subunit meets the left side of the central protuberance.
	It is not certain whether areas A and B are continuous or interrupted. Areas C and D are disconnected. Areas A and B are separated by the tunnel from areas C and D. Location of mRNA The mRNA binding region is located on the right temporal side of the head of the 30S subunit. This region corresponds to area B and is in contact with the 50S subunit. The messenger RNA molecule passes between the right temporal region of the 30S subunit and the central protuberance of the large subunit. The mRNA strand in this region is protected from the action of nucleases. In the case of natural mRNA, the protected region consists of 35-45 nucleotides

Location of tRNA
Transfer RNA is also protected from nuclease action, and it is therefore likely that both peptidyl and aminoacyl tRNAs lie in a cavity in between the two subunits of the ribosome. Rich (1974) has suggested that there are two channels, an entrance channel for aminoacyl tRNA and an exit channel for tRNA.

It has been suggested that the anticodon loop of tRNA is located in the mRNA binding domain and the -CCA end in the region of proteins L2, L16 and L27. The growing polypeptide chain is also located near L2, Ll6, L27.

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