ABSTRACTMultiple essential small GTPases are involved in the assembly of the ribosome or in the control of its activity. Among them, ObgE (CgtA) has been shown recently to act as a ribosome antiassociation factor that binds to ppGpp, a regulator whose best-known target is RNA polymerase. The present study was aimed at elucidating the expression ofobgEinEscherichia coli. We show thatobgEis cotranscribed with ribosomal protein genesrplUandrpmAand with a gene of unknown function,yhbE. We show here that about 75% of the transcripts terminate beforeobgE, because there is a transcriptional terminator betweenrpmAandyhbE. As expected for ribosomal protein operons, expression was highest during exponential growth, decreased during entry into stationary phase, and became almost undetectable thereafter. Expression of the operon was derepressed in mutants lacking ppGpp or DksA. However, regulation by these factors appears to occur post-transcription initiation, since no effects of ppGpp and DksA onrplUpromoter activity were observedin vitro.IMPORTANCEThe conserved and essential ObgE GTPase binds to the ribosome and affects its assembly. ObgE has also been reported to impact chromosome segregation, cell division, resistance to DNA damage, and, perhaps most interestingly, persister formation and antibiotic tolerance. However, it is unclear whether these effects are related to its role in ribosome formation. Despite its importance, no studies on ObgE expression have been reported. We demonstrate here thatobgEis expressed from an operon encoding two ribosomal proteins, that the operon's expression varies with the growth phase, and that it is dependent on the transcription regulators ppGpp and DksA. Our results thus demonstrate thatobgEexpression is coupled to ribosomal gene expression.
The yeast omnipotent suppressor SUP46 encodes a ribosomal protein which is a functional and structural homolog of the Escherichia coli S4 ram protein.
