YihI activates the GTPase activity of Der, a 50S ribosomal subunit stability factor. The stimulation is specific to Der as YihI does not stimulate the GTPase activity of Era or ObgE. The interaction of YihI with Der requires only the C-terminal 78 amino acids of YihI. A yihI deletion mutant is viable and shows a shorter lag period, but the same post-lag growth rate as a wild-type strain. yihI is expressed during the lag period. Overexpression of yihI inhibits cell growth and biogenesis of the 50S ribosomal subunit. YihI is an unusual, highly hydrophilic protein with an uneven distribution of charged residues, resulting in an N-terminal region with high pI and a C-terminal region with low pI.