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When stressed, all cells downregulate protein synthesis, to conserve energy and shift resources towards repair. Here, we show that in some mammalian cells, including neurons, stress also results in the formation of inactive ribosome-ribosome clusters (disomes). Using cryogenic electron tomography we visualized ribosomes in situ and observed that this dimerization is mediated by a homotypic interaction of rRNA expansion segment ES31Lb. ES31Lb interactions are both necessary and sufficient for disome formation and confer a growth advantage and stress-resistance to cells. ES31Lb is predicted to homo-dimerize in ~20% of chordates, including both a chicken and human variants. Cryo-ET analysis of long-studied chicken tetrasomes revealed an interaction between ES31Lb and ES9La. Taken together, these data indicate a mechanism of translation regulation in animal cells that operates using a flexible component of the protein synthesis machinery - rRNA expansion segments.