A I~ during chloramphenicol inhibition ('chloramphenicol particles') have been isolated by large-scale rate-zonal density gradient ultracentrifugation. The proteins and RNA species composing these particles have been examined.The rRNA species present are precursor and mature forms of 16s and 23s rRNA which accumulate during inhibition. The proteins prepared directly from the particles give strong multiple irnmunoprecipitates with antisera specific to 30 S and 50s ribosomal proteins. The soluble proteins of the cell prepared in the same manner do not give this immunological reaction. Two-dimensional electrophoresis patterns of the proteins from the 'chloramphenicol particles' strongly resemble those for 30s and 50s ribosomal proteins, i.e. they are predominantly basic low molecular weight proteins, and are dissimilar to the patterns for the soluble proteins of the cell.It is concluded that the 'chloramphenicol particles' are a heterogeneous group of ribonucleoproteins comprising the bulk of the rRNA accumulating during inhibition in association with variable amounts of some of their corresponding ribosomal proteins. The particles are therefore not artefacts of preparation, as previously thought, but arrested ribosome precursors.

The ' relaxed particles ' formed during methionine starvation of Escherichia coli A I~ (Hfr re1 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 I 6 S and 23 S 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 30 S and 50 S 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 relation to ribosome biogenesis and the stabilization of rRNA.