Dimerization and inactivation of ribosomes in Escherichia coli is a two-step process that involves the binding of ribosome modulation factor (RMF) and hibernation promotion factor (HPF). Lactococcus lactisMG1363 expresses a protein, YfiA(Ll), which associates with ribosomes in the stationary phase of growth and is responsible for dimerization of ribosomes. We show that full-length YfiA(Ll) is necessary and sufficient for ribosome dimerization in L.lactis but also functions heterologously in vitro with E.coli ribosomes. Deletion of the yfiA gene has no effect on the growth rate but diminishes the survival of L.lactis under energy-starving conditions. The N-terminal domain of YfiA(Ll) is homologous to HPF from E.coli, whereas the C-terminal domain has no counterpart in E.coli. By assembling ribosome dimers in vitro, we could dissect the roles of the N- and C-terminal domains of YfiA(Ll). It is concluded that the dimerization and inactivation of ribosomes in L.lactis and E.coli differ in several cellular and molecular aspects. In addition, two-dimensional maps of dimeric ribosomes from L.lactis obtained by single particle electron microscopy show a marked structural difference in monomer association in comparison to the ribosome dimers in E.coli.