Analysis of the effect of ppGpp on the ftsQAZ operon in Escherichia coli.

Escherichia coli loses its rod shape by inactivation of PBP2 (penicillin-binding protein 2), target of the beta-lactam mecillinam. Under these conditions, cell division is blocked in rich medium. Division in the absence of PBP2 activity is restored (and resistance to mecillinam is conferred) when the three cell division proteins FtsQ, FtsA and FtsZ are overproduced, but not when only one or two of them are overproduced. Division in the absence of PBP2 activity is also restored by a doubling in the ppGpp pool, as in the argS201 mutant. However, the nucleotide ppGpp, a transcriptional regulator of many operons, does not govern any of the five promoters of the ftsQAZoperon, as shown by S1 mapping of ftsQAZ mRNA 5' ends in exponentially growing wild-type cells in the mecillinam-resistant argS201 mutant (intermediate ppGpp level) or during the stringent response elicited by isoleucine starvation (high ppGpp level). Furthermore, the concentration of FtsZ protein is not increased in exponentially growing mecillinam-resistant argS201 cells. These results show that the ftsQAZ operon is not the ppGpp target responsible for mecillinam resistance. We are currently trying to identify those targets that, at intermediate ppGpp levels, allow cells to divide as spheres in the absence of PBP2.