Abstract
Throughout nature, ribosomes are found to be composed of two subunits, one approximately twice the size of the other. Whereas the function of ribosomes as the complexes active in the synthesis of growing polypeptide chains has been long recognized, a functional basis for the universal bipartite construction of ribosomes has only recently been demonstrated by the finding that in vivo the 30 S and 50 S subunits of bacterial ribosomes are not permanently associated. Instead, ribosomes in growing Escherichia coli frequently undergo exchange of their 30 S and 50 S subunits, apparently by dissociation and reformation from a pool of free subunits that continuously recycle through ribosomes. This exchange was demonstrated by Kaempfer, Meselson, and Raskas (1968) by density gradient analysis of ribosomes following transfer of a bacterial culture from a heavy- to a light-isotope medium. Kinetic studies consistent with such exchange have been reported by Mangiarotti and Schlessinger (1967).
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