It has been known for some time that increased amounts of FtsZ protein have dramatic effects on cell division (4, 17). Increasing amounts of FtsZ (up to seven times the normal level) cause increasing numbers of additional septa to form near the cell poles, producing minicells. A plasmid, pZAQ, carrying the complete ftsQ, ftsA, and ftsZ genes (Fig. ​(Fig.1)1) increased FtsZ levels by about sevenfold and increased the frequency of both polar and centrally located septa (4). The increase in both types of septa was originally attributed to increased FtsZ (4), but our new results show that an increase in both FtsZ and FtsA is required to cause early central division.

(Interestingly, in the experiments reported by Ward and Lutkenhaus [17] an increase in central divisions was not seen in cells containing a plasmid, pJW5, which has a deletion in the beginning of the ftsQ gene but carries intact ftsA and ftsZ genes. This plasmid overproduced FtsZ to a level comparable to that by pZAQ but failed to increase the frequency of central divisions. This result therefore seemed to contradict both the conclusion of Ward and Lutkenhaus [17], that increased FtsZ alone can reduce the length of cells at division, and that of the present paper, that increased FtsA plus FtsZ is sufficient to do this. We think that the explanation for this anomaly is that the ftsA gene is not expressed, or at least not expressed at a high level, from pJW5, because we have earlier shown that deletion of the upstream part of ftsQ, at the same BamHI site used to construct pJW5, abolishes the expression of the downstream ftsA gene [9]. Overexpression of ftsZ would still take place in pJW5 because of transcription from promoters that lie within ftsA [13, 18]. In the corresponding plasmid used in this work, pZAQ*, both ftsA and ftsZ are expressed because we used a frameshift mutation, rather than a deletion, to inactivate ftsQ. Table ​Table11 shows that this plasmid is indeed capable of inducing early central division in wild-type cells.)

We have previously suggested (10, 14) that cell poles represent used division sites. Such sites can be reactivated by newly synthesized FtsZ protein (17), although this is normally prevented by the action of the Min proteins (7). Overproduction of FtsZ by itself can overcome the inhibitory action of the Min proteins and allow septa to form at the cell poles (4, 17), but overproduction of FtsA cannot. In contrast, as we show here, coordinate overproduction of FtsA and FtsZ together not only allows the use of cell poles for septation but also enables new, nonpolar septation sites to be activated earlier in the cell cycle (Table ​(Table1).1). In minB mutants, polar sites and newly formed sites compete equally for a limited amount of division potential (10, 14), so that minicells form at the expense of normal divisions. Bi and Lutkenhaus (5) showed that division potential can be increased in minB mutants by increasing the production of FtsZ and FtsA but not by increasing FtsA alone. We have shown here that an increase in FtsZ alone also does not increase the capacity of cells to make new septa at internal (nonpolar) sites and that, to do this, coordinate overexpression of both FtsA and FtsZ is required. Therefore, division potential, or the capacity of a cell to make a particular number of septa at new division sites, is limited by both FtsA and FtsZ proteins.

Rings of FtsZ can form at division sites in the absence of functional FtsA, but these rings are unable to contract (1). FtsA also forms rings at division sites but only in the presence of an FtsZ ring (2). The experiments that we report here may be understood if a critical ratio of FtsA to FtsZ is required for the function of the FtsA-FtsZ ring, as previously suggested (6, 8).

Further studies are required to find out how FtsA and FtsZ interact and how it is that excess FtsZ can apparently reactivate old division sites but requires an additional increase in FtsA in order to activate new sites. One possibility is that FtsA is sequestered in old division sites, as has been suggested by the finding that FtsA protein must be remade in every cycle (11), and that some temperature-sensitive mutant forms of FtsA can cause long-lasting inactivation of division sites formed at the restrictive temperature (15). Alternatively, perhaps the entire FtsA-FtsZ complex is sequestered at the cell poles but blocked by the interaction between MinC and FtsZ; production of excess FtsZ might then titrate out the MinC and release the activity of the sequestered division proteins.