## 13 Does DnaA or a DnaA-Based Initiation Hyperstructure Also Trigger Division? DNA replication is clearly coupled to cell division, insofar as signalling systems exist to prevent division when DNA has been damaged [83]. These include the SOS system that, when induced by DNA damage, produces the SuLA/SfiA protein (along with forty other proteins) to interfere with the action of the key protein in cell division, FtsZ [84]. Using synchronised populations of _E. coli_, it was found that the levels of _ftsZ_ mRNA increase at the time of initiation of replication [85; 86]. That said, different results have also been obtained [87]. Significantly, the 2 min, or _dcw_, cluster of genes in _E. coli_ contains three DnaA boxes upstream of _ftsZ_ (within _ftsQA_) that were, however, not found to affect _ftsZ_ expression in the conditions tested [86; 88]; their role, therefore, remains an intriguing, open question. For example, could these boxes serve to connect physically, via a DnaA polymer, the 2 min transcription stage of the division hyperstructure with the initiation hyperstructure? In _B. subtilis_, DnaA binds the promoter region of _ftsL_, which encodes a key cell divisionprotein [89], whilst, in _C. crescentus_, DnaA binds in vitro to the promoter region of _ftsZ_, which is believed to be part of the DnaA regulon that coordinates the initiation of DNA replication with cell cycle progression [90,91]. In _E. coli_, the putative coupling of DNA replication to cell division is also supported by the fact that several _ts_ mutants affected in the initiation or elongation steps of DNA replication stop dividing normally at a non-permissive temperature and form filamentous cells that resemble those formed by a _thy_ mutant during thymine starvation [92]. This cessation of division does not, however, necessarily mean that some aspect of the replication of the chromosome (including termination of replication) is responsible for the initiation of cell division. Indeed, after further cultivation of _ts_ replication mutants at the non-permissive temperature, division resumes towards the ends of the filamentous cells to produce cells that lack chromosomal DNA; these _anculate_ cells are of almost normal size [92,93]. This production of anucleate cells occurs with _dnaA_, _dnaC_, _dnaG_ (_parB_), and _dnaB ts_ mutants and requires the absence of the inhibitor of division, SulA/SfiA, and a mutation in _ftsZ_ (_sfiB_) [92]. How might this production occur? It may be significant that the above anucleate cell production also requires cAMP (via either the activity of the wild type _cya_ gene or an exogenous supply of cAMP) along with the cAMP receptor protein, CAP [92]. CAP regulates the transcription of over 100 genes in _E. coli_, including those in the _lac_ operon. It is therefore conceivable that (1) there are major differences in the structure of the membrane in the presence and absence of cAMP and CAP and (2) these differences could affect transertion (e.g., via Lac permease) and, hence, the membrane domain dynamics that are proposed to time and position division [94,95]. If membrane dynamics do, indeed, underpin the regulation of the cell cycle at a fundamental level, it would make sense for proteins such as DnaA to respond to this fundamental system too, given that these sophisticated proteins presumably evolved some time after protocells had achieved some mastery over replication and division. It could also be argued that it would have made sense for the earliest protocells to have had the same signalling mechanism leading to both DNA replication and cell division. This is because the RNA and/or DNA in these protocells was probably short and, in the _Ring World_ scenario, in the form of a population of ds RNA/DNA rings, each of which catalysed a different reaction [53,96]; hence, the fundamental problem that protocells had to solve was not how to divide after replicating a long stretch of DNA, but rather how to proceed successfully through a complete cell cycle, which is a single decision.