L actococcus lactis   YfiA is necessary and sufficient for ribosome dimerization

Dimerization and inactivation of ribosomes in E scherichia coli is a two‐step process that involves the binding of ribosome modulation factor ( RMF ) and hibernation promotion factor ( HPF ). L actococcus lactis MG 1363 expresses a protein, YfiA L l , which associates with ribosomes in the stationary phase of growth and is responsible for dimerization of ribosomes. We show that full‐length YfiA L l 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 L l 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 L l . 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 .