The nature of the proteins present in the 'relaxed particles' from methionine-starved Escherichia coli A19 (Hfr rel met rns)

Abstract

The 'relaxed particles' formed during methionine starvation of Escherichia coli A19 (Hfr rel met rns) have been isolated by large-scale rate-zonal density gradient ultracentrifugation. The proteins and rRNA species associated with these particles have been examined. The rRNA species present are precursor and mature forms of 16S and 23S rRNA. The bulk of the rRNA which accumulates during starvation is found within the particles. The proteins prepared directly from the particles give strong multiple immunoprecipitates with antisera specific to 30S and 50S ribosomal proteins. The soluble proteins, prepared and examined in the same manner, do not give this immunological reaction. Two-dimensional electrophoresis patterns of the proteins from the particles show that the proteins co-migrate with proteins from 30S and 50S ribosomes and are entirely dissimilar to the proteins prepared by the same methods from the soluble fraction of the cells. On the basis of these and other observations, it is concluded that the 'relaxed particles' are not artefacts but are arrested ribosome precursors containing both rRNA and certain ribosomal proteins. The free pool of ribosomal proteins is low in exponential-phase cells and is not significantly increased by a 2 h period of starvation for glucose. The implications of these observations concerning the proteins associated with 'relaxed' and 'chloramphenicol particles' are discussed in raltion to ribosome biogenesis and the stabilization of rRNA.