![]() The graph on the right indicates whether the mentioned species has a dimorphic life cycle, as defined by a differential reproductive state and size of the two daughter cells. Shown is a stylized phylogenetic tree of several representatives of the Alphaproteobacteria. To enable their complex life cycles, these species have taken up the challenging task to establish an asymmetry between the two poles of predivisional cells that translates into different morphologies and fates in the two daughter cells.ĭimorphism within the Alphaproteobacteria correlates with the essentiality of the CtrA regulatory network as well as the presence of the DivK polarity and the CtrA proteolysis modules. 1990) and Hyphomicrobiaceae ( Moore 1981), and binary fission combined with polar stalk formation in the Caulobacteraceae ( Pointdexter 1964). 2012), stalk-terminal budding in the Hyphomonadaceae ( Moore et al. Interestingly, concomitant with the establishment of cellular asymmetry, multiple different growth modes have evolved within this lineage, such as polar growth in the Rhizobiales ( Brown et al. These asymmetric, dimorphic life cycles appear to have arisen in the orders Rhizobiales and Caulobacterales after they had diverged from the Rhodobacterales ( Muñoz-Gómez et al. After birth, one of them is immediately ready to enter S phase and initiate a new round of DNA replication, whereas its sibling shows a pronounced lag (G1 phase) before it starts its reproductive cycle ( Degnen and Newton 1972 Ehrle et al. Various members of this class show a striking asymmetry, in which the physiological and reproductive state of the two siblings differ markedly after cell division ( Figure 1). A prominent example are the Alphaproteobacteria, which include intracellular pathogens, plant-associated bacteria as well as free-living species. ![]() Whereas many bacteria grow and divide symmetrically, some bacteria have evolved more complex modes of reproduction. ![]() In this review, we summarize the current knowledge of the CtrA pathway and discuss how it has evolved to regulate the cell cycle of morphologically distinct alphaproteobacteria. Phase-separated polar microdomains of changing composition sequester proteins involved in the (in-)activation and degradation of CtrA specifically at each pole. CtrA is differentially activated in the two (developing) offspring, thereby establishing distinct transcriptional programs that ultimately determine their distinct cell fates. The DNA-binding activity of CtrA is controlled at the level of both protein phosphorylation and stability, dependent on an intricate network of regulatory proteins, whose function is tightly coordinated in time and space. At the center of this network is the essential DNA-binding response regulator CtrA, which acts as a transcription factor controlling numerous genes with cell cycle-relevant functions as well as a regulator of chromosome replication. To establish this asymmetry, these species employ a complex cell cycle regulatory pathway based on two-component signaling cascades. While many bacteria divide by symmetric binary fission, some alphaproteobacteria have strikingly asymmetric cell cycles, producing offspring that differs significantly in their morphology and reproductive state. ![]()
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