Required for dimerization of active 70S ribosomes into 100S ribosomes in stationary phase; 100S ribosomes are translationally inactive and sometimes present during exponential growth. May not be the only factor implicated (PubMed:22950019). Might negatively regulate the activity of the sigma-54 factor (SigL) (PubMed:9852014). Interacts with 100S ribosomes. Not associated with 70S ribosome monomers, about 1 monomer per ribosome (PubMed:22950019). Expressed under the dual control of sigma-B (PubMed:9296790) and sigma-H factors (PubMed:9296790, PubMed:9852014). The sigma-B-dependent promoter drives expression of yvyD under stress conditions and after glucose starvation, whereas a sigma-H-dependent promoter is responsible for yvyD transcription following amino acid depletion (PubMed:9852014). Also claimed to be under sole control of sigma-H (PubMed:22950019). The expression of a sigL-dependent gene (levD) is up-regulated two-fold in cells lacking yvyD, but the relative amount of the sigL transcript is not increased (PubMed:9852014). In cells overexpressing ppGpp synthase YwaC, deletion of this gene causes loss of 100S ribosome formation by dimerizing 70S ribosomes (PubMed:22950019). Belongs to the HPF/YfiA ribosome-associated protein family. Long HPF subfamily.