枯草菌における胞子形成

図1. 枯草菌の胞子形成過程

枯草菌（Bacillus subtilis）は、土壌や植物に自然に存在し、小さくて丈夫で代謝的に休眠したエンドスポアを形成する能力で知られる、桿菌であり、グラム陽性のバクテリアである。枯草菌は対称的に分裂して2つの娘細胞（二分裂）を作ることも、非対称的に1つの環境要因（熱、乾燥、放射線、化学的攻撃など）に耐性のあるエンドスポアを生成することもできる。これは長期間環境中に持続することができます。エンドスポアは栄養ストレス時に形成され、生物が環境中で生き延びることを可能にする。エンドスポア形成のプロセスは、2つの娘細胞の根本的な形態と生理学的な変化をもたらす。このプロセスは、最終的には1つの娘細胞（スポア）の代謝活動の停止と他の娘細胞（‘母細胞’）の死に伴う過程である。

概要

Commitment to sporulation

Although sporulation in B. subtilis is induced by starvation, the sporulation developmental program is not initiated immediately when growth slows due to nutrient limitation. A variety of alternative responses can occur, including the activation of flagellar motility to seek new food sources by chemotaxis, the production of antibiotics to destroy competing soil microbes, the secretion of hydrolytic enzymes to scavenge extracellular proteins and polysaccharides, or the induction of ‘competence’ for uptake of exogenous DNA for consumption, with the occasional side-effect that new genetic information is stably integrated. Sporulation is the last-ditch response to starvation and is suppressed until alternative responses prove inadequate. Even then, certain conditions must be met such as chromosome integrity, the state of chromosomal replication, and the functioning of the Krebs cycle.

Nature of regulation

Sporulation requires a great deal of time and also a lot of energy and is essentially irreversible, making it crucial for a cell to monitor its surroundings efficiently and ensure that sporulation is embarked upon at only the most appropriate times. The wrong decision can be catastrophic: a vegetative cell will die if the conditions are too harsh, while bacteria forming spores in an environment which is conducive to vegetative growth will be out competed. In short, initiation of sporulation is a very tightly regulated network with numerous checkpoints for efficient control.

Control points

Two transcriptional regulators, σH and Spo0A, play key roles in initiation of sporulation. Several additional proteins participate, mainly by controlling the accumulated concentration of Spo0A~P. Spo0A lies at the end of a series of inter-protein phosphotransfer reactions, Kin–Spo0F–Spo0B–Spo0A, termed as a ‘phosphorelay’. The regulation of these various factors controlling the accumulated concentration of Spo0A~P, and their interactions are described in detail in Figure2:

Fig2. Gene Regulatory Network for Initiation of Sporulation in Bacillus subtilis

In the table below, the term 'Activators' refers to genes/proteins that ultimately result in initiation of sporulation and 'Repressors' refers to the ones that inhibit this initiation of sporulation.

Activators	Regulation	Repressors	Regulation
KinA	Transfers phosphate to the Spo0F	Sda	Blocks autophosphorylation of KinA
KinB	Transfers phosphate to the Spo0F	KipI	Blocks autophosphorylation of KinA
Spo0A	Activates several key sporulation-specific genes	Spo0A	Negatively controls transcription of abrB
Spo0H (σH)	Activates phrE gene	ComA	Activates competence
Spo0B	Phosphotransferase initiation	SinR	Negative regulation of kinB
Spo0F	Phosphotransferase initiation	RapA, RapB, RapE, and RapH	Dephosphorylation of Spo0F~P
ComA	Activates phrA	Spo0E, YisI, and YnzD	Dephosphorylation of Spo0A~P
SinI	Antagonist of SinR	GTP-bound CodY	Inhibits rapA-phrA
KipA	Inhibits kipI gene	AbrB	Encodes a repressor of spo0H
PhrA	Suppresses dephosphorylation activity of RapA	Hpr	Overexpression inhibits sporulation
PhrE	Inhibits RapE	DnaA	Over expression of Sda
PhrH	Inhibits RapH

Homologous networks in eukaryotes

Spores form a part of the life cycles of a diverse range of organisms such as many bacteria, plants, algae, fungi and some protozoa. In Saccharomyces cerevisiae (Kingdom Fungi), the set of early genes activating sporulation is induced by Ime1 (Inducer of Meiosis 1) and a regulator of middle genes is Ndt80p.

See also

• Bacillus
• Sporophyte
• Bacillus subtilis

External links

• SubtiList - GenoList - Institut Pasteur
• SubtilisWiki (archived)
• Bacillus subtilis - Parts Registry
• Video - Sporulation in Bacillus subtilis