Fusidic acid-resistant EF-G perturbs the accumulation of ppGpp

Reductions in growth rate caused by fusidic acid-resistant EF-G mutants in Salmonella typhimurium correlate strongly with increased mean cell size. Th is is unusual because growth rate and cell size normally correlate positive ly. The global transcription regulator molecule ppGpp has a role in co-ordi nating growth rate and division, and its basal level normally correlates in versely with cell size at division. We show that fusidic acid-resistant EF- G mutants have perturbed ppGpp basal levels during steady-state growth and perturbed induced levels during starvation. One mutation, fusA1, associated with the slowest growth rate and largest cell size, causes a reduction in the basal level of ppGpp to one-third of that found in the wild-type strain . Other fusA mutants with intermediate or wild-type growth rates and cell s izes have either normal or increased basal levels of ppGpp. There is an inv erse relationship between the basal level of ppGpp in vivo and the degree t o which translation dependent on mutant EF-G is inhibited by ppGpp in vitro . This enhanced interaction between mutant EF-G and ppGpp correlates with a n increased K-M for GTP. Our results suggest that mutant EF-G modulates the production of ppGpp by the RelA (PSI) pathway. In conclusion, fusidic acid -resistant EF-G mutations alter the level of ppGpp and break the normal rel ationship between growth rate and cell size at division. It would not be su rprising if other phenotypes associated with these mutants, such as loss of virulence, were also related to perturbations in ppGpp levels effected thr ough altered transcription patterns.