Localization of Ribosome Formation in Escherichia Coli

Abstract

A number of ribonucleoprotein fractions have been isolated from Escherichia coli K 12, one of which is more strongly bound to the cell membrane than the rest and can be detached only by deoxycholate treatment. Several properties have been analyzed. The following are common to all fractions: Sedimentation characteristics of sub-units; Sedimentation constant, nucleotide composition and capacity to hybridize with DNA of RNA; electrophoretic behaviour of proteins. Other properties differ in the various fractions: cell membrane fractions have higher RNA/protein ratio, are more sensitive to RNAase and dissociate more easily into two subunits. By examining the rate of incorporation of [2-¹⁴C]-uracil into the various fractions, it has been found that cell membrane-bound ribonucleoprotein fractions contain a higher proportion of newly formed rRNA and part of this early-labeled rRNA is contained in particles sedimenting as ribosome precursors. Moreover, by isolating ribonucleoprotein fractions from cells grown in the presence of chloramphenicol, cell membrane-bound riconucleoprotein fraction is richer in CM particles. All these results lead to the conclusion that this fraction contains a higher proportion of ribosome precursors. The meaning of these results is discussed.