Figure 6.

Proposed model for a critical cell size that depends on mRNA transcript abundance during steady-state growth. We suggest a critical cell size above which increasing size (X-axis, directly proportional to protein and ribosome abundance) begins to compete with the optimal growth rate in steady-state conditions (Y-Axis). Given a constant level of mRNA transcripts, by how much can cell size increase while maintaining optimized balanced growth? To maintain optimal growth, the ribosomal fraction scales with the abundance of proteins. As long as it is low enough, the fraction of translating ribosomes can also be maintained, as sufficient mRNA is available. In this regime, the growth rate is not affected by the change in total protein levels. However, further increasing protein abundance beyond this size, necessarily reduces the fraction of co-translating ribosomes, leading to a reduction in cell growth rate. See the text for details.