A GTPase-activating protein (GAP) that modifies Der/EngA GTPase function, negatively regulating cell growth, probably via ribosome assembly. Stimulates the GTPase activity of Der; a construct missing the first 45 residues is even more stimulatory. Does not stimulate 2 other GTPases (ObgE and Era). Overexpression inhibits cell growth; precursor 16S rRNA accumulates, the 23S rRNA is 6-7 bases longer than usual, and 50S ribosomal subunits are improperly assembled, leading to 45S subunits lacking proteins L9, L18 and L25. Overexpression of Der in the same cells suppresses the 50S subunit assembly defect, corroborating that YihI and Der interact. Homodimer. Interacts with Der/EngA via the last 78 residues. Interaction with Der occurs at a 1:1 stoichiometry, suggesting the dimer dissociates to interact with Der. Highly expressed during the lag phase, not expressed in later phases. Dispensable for cell growth, cells lacking this gene have a shorter lag phase. This is a highly hydrophilic protein; there is a stretch of highly charged residues in the N-terminus and a stretch of acidic residues in the C-terminus. Belongs to the YihI family.