Ribosomes generate all proteins in a cell, including their own ribosomal proteins. Previous work on bacteria showed that ribosomal protein production imposes a bound on cellular growth rates, since cell doubling requires a commensurate doubling of ribosomes. However, ribosomes are made not only of protein, but also of ribosomal RNA. We obtain a new speed limit on cell growth which originates in the generation of ribosomal RNA . A comparison with E. coli data reveals that the bacterial ribosome's 1:2 protein-to-RNA mass ratio uniquely maximizes cellular growth rates as permitted by both bounds. This observation leads to a growth-law involving RNA polymerases, and an invariant of bacterial growth. Similar arguments for Eukarya lead to several new growth-laws . Despite the greater complexity of that domain of life, the predictions are consistent thus far with available data for the model organism S. cerevisiae.
 S. Kostinski and S. Reuveni, "Ribosome composition maximizes cellular growth rates in E. coli," Phys. Rev. Lett. 125, 028103 (2020).
 S. Kostinski and S. Reuveni, "Growth-laws and invariants from ribosome biogenesis in lower Eukarya," arXiv:2008.11697 (2020).