Ribosome modulation factor (RMF) is a protein specifically associated with 100S ribosome dimers which start to accumulate in Escherichia coli cells upon growth transition from exponential to stationary phase. The structural gene, rmf, encoding the 55 amino acid residues RMF protein has been cloned from the 21.8 min region of the E. coli genome and sequenced. While rmf was silent in rapidly growing exponential phase cells, a high level of transcription took place concomitantly with the growth transition to stationary phase. Under slow growth conditions, rmf was expressed even in exponential phase and there was an inverse relationship between the expression of rmf and the cell growth rate. Thus, the expression profile of rmf is contrary to those of genes for ribosomal components and ribosome-associated proteins constituting the translational apparatus. The katF gene product, a stationary phase-specific sigma factor, was not required for the expression of rmf. Disruption of rmf resulted in loss of ribosome dimers and reduction of cell viability during stationary phase.