Читать книгу Structure and Function of the Bacterial Genome - Charles J. Dorman - Страница 36

The cell division septum must be placed centrally if rod‐shaped bacteria like E. coli are to divide into daughter cells of equal size. This placement is achieved through a gradient of FtsZ‐inhibitor Min proteins. The gradients extend from regions of maximum Min density at the cell poles to a region of minimum density (and therefore minimum FtsZ inhibition) at mid‐cell (Bramkamp and van Baarle 2009; Monahan and Harry 2012; Rowlett and Margolin 2015). While nucleoid occlusion operates to prevent guillotining of the nucleoid by the closing division septum, the Min system works independently of the nucleoid and is concerned with the correct localisation of the septum. Indeed, bacteria that are rendered chromosomeless still tend to form the cell division septum at the mid cell (Sun et al. 1998).

The term ‘Min’ is derived from ‘minicell’, a phenotype in E. coli min mutants (and other rod‐shaped bacteria) where eccentric placement of the division septum produces two daughter cells of uneven length, one of which is too small to accommodate a chromosome (Adler et al. 1967; Reeve et al. 1973), although it can house plasmids (Roozen et al. 1971).

E. coli uses the MinC protein to inhibit FtsZ polymerisation (Hu et al. 1999; Hu and Lutkenhaus 2000) but, unlike B. subtilis, it lacks a cell‐pole‐anchoring protein that can be used to recruit MinC and other Min complex components to that part of the envelope. It relies instead on a MinC protein gradient extending from each pole to the midcell, with MinC forming a complex with the membrane‐binding MinD ParA‐like ATPase protein (de Boer et al. 1989, 1991; Hu and Lutkenhaus 2003). A third protein, MinE, is used to target the MinCD complex to the cell poles, with MinE (and phospholipid) stimulating the ATPase activity of MinD (Hu and Lutkenhaus 2001). MinE binds to the membrane at the pole, targeting MinCD complexes, displacing both MinC and MinD and stimulating ATP hydrolysis by MinD (Loose et al. 2011; Park et al. 2011). MinE and MinD set up a high‐speed oscillating system in which MinC is trafficked from pole to pole, on average spending a minimum of time at mid‐cell and most of the time at the poles (Raskin and de Boer 1997; Hu and Lutkenhaus 1999; Hu et al. 2002). It is the relative paucity of MinC at mid‐cell that diminishes the inhibitory influence on FtsZ polymerisation and Z‐ring formation (Hu and Lutkenhaus 1999; Raskin and de Boer 1999a,b). In addition to inhibiting FtsZ polymerisation by protein‐protein interaction, the oscillation of MinC populations from pole to pole has an impact on the distribution of other FtsZ‐interactors. Together with FtsZ itself, the ZapA, ZapB, and ZipA proteins oscillate oppositely to MinC and with a similar dynamic pattern. ZapB does not bind FtsZ directly but through ZapA, which does bind FtsZ. ZipA, with FtsA, connects FtsZ to the cytoplasmic membrane (Pichoff and Lutkenhaus 2005) while ZapA‐ZapB stimulates Z‐ring formation and stabilises it (Buss et al. 2013; Galli and Gerdes 2010; Gueiros‐Filho and Losick 2002). Therefore, the oscillatory movements of MinC proteins probably trigger periodic assembly and disassembly of the Z ring complexes (Bisicchia et al. 2013; Thanedar and Margolin 2004).

B. subtilis possesses the cell‐pole‐targeting protein DivIVA, which is involved both in chromosome attachment at the pole in sporulating cells (Section 1.10) and in directing the cellular localisation of MinC (Cha and Stewart 1997; Edwards and Errington 1997). The utility of DivIVA as a general pole‐targeting protein arises from its ability to sense cell membrane curvature, which is maximal at the poles (Edwards et al. 2000; Lenarcic et al. 2009). The MinC protein is bound by MinD and an adaptor protein, MinJ, connects this complex to DivIVA (Bramkamp et al. 2008; Patrick and Kearns 2008). As the division septum develops, invagination of the membrane, and the associated membrane curvature, recruit DivIVA from the pole to the mid‐cell (which is the soon‐to‐be pole of the new daughter cell) (Gregory et al. 2008; Rodriguez and Harry 2012; van Baarle and Bramkamp 2010).

Genetic elimination of the Min system and of nucleoid occlusion is deleterious for cell growth in rich medium, but the mutants can grow and divide in minimal medium (Bernhardt and de Boer 2005; Yu and Margolin 1999). This suggests that additional systems exist to ensure chromosome segregation and cell division (Bailey et al. 2014; Cambridge et al. 2014). A link between the Ter‐matS‐binding MatP protein and ZapB connects the Ter macrodomain of the chromosome to the divisome's ZapB‐ZapA‐FtsZ complex (Espéli et al. 2012) and this may afford the nucleoid itself a role in determining Z ring placement (Rowlett and Margolin 2015; Yu and Margolin 1999).

The Min system operates on Z ring placement through an inhibitory mechanism. This strategy is not used universally among bacteria. For example, positive placement is used to direct Z ring positioning in Streptomyces spp. Here, the SsgA protein takes up position at mid‐cell and recruits a partner, SsgB, which is thought to promote FtsZ polymerisation and to link the Z rings to the membrane. These rings form the basis of sporulation septa in these actinomycetes (Traag and van Wezel 2008; Willemse et al. 2011). Sporulation takes place in the aerial hyphae where up to one hundred division septa are laid down, dividing the hyphae into compartments that each contain one genome copy (Zhang, L. et al. 2016). The SepG membrane protein recruits SsgB to future septum sites; it is also required for nucleoid compaction indicating that plays a role in coordinating chromosome organisation and placement of the division septum (Zhang et al. 2016). The PomZ protein in Myxococcus spp. is also a positive regulator of Z‐ring positioning; PomZ shares with MinD the property of being a ParA‐like ATPase (Treuner‐Lange et al. 2013). It forms a complex with PomX and PomY that moves over the surface of the nucleoid in a biased, randomised motion that becomes constrained at the mid‐cell. Once at the mid‐cell location, PomXYZ recruits FtsZ (Schumacher et al. 2017).

Macrodomains play important roles in determining the choreography of the daughter chromosomes, as these segregate prior to cell division (Espéli et al. 2008). They also correlate with global gene expression patterns, suggesting that the overall gene expression programme of the cell is written into the architecture of the nucleoid (Cameron et al. 2017; Sobetzko 2016; Sobetzko et al. 2012). To appreciate the significance of the connections between nucleoid structure and gene expression, it will be necessary to consider the contributions made to both by variable DNA structure and NAPs.