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Index >> Bacterial Structure >> Endospore Formation Stage 4 - Cortex Synthesis

Endospore Formation Stage 4 - Cortex Synthesis

	Endospore Formation Stage 4 - Cortex Synthesis- The cortex develops between the inner and the outer forespore membranes. In B. sphaericus this occurs in two stages, deposition of the primordial cell wall and deposition of spore-specific peptidoglycan(cortex) outside the primordial cell wall. The forespore also begins to accumulate calcium and pyridine-2, 6-dicarboxylic acid (DP A) during stage IV, and becomes grey in appearance. At the end of stage III there is rapid increase in the accumulation of Ca++ from the sporulation medium into the cell.
The accumulated Ca++ moves continually into the fore spore compartment, so that in the mature spore it consists of about 2% of the dry- weight of the spore. The shift from Ca++ efflux of vegetative bacilli to Ca++ influx during sporulation may require synthesis of new catalytic membrane proteins, or a modification of the vegetative efflux mechanism. Alternatively, the influx may be by facilitated diffusion, rather than by active transport. Experimental evidence indicates that Ca++ is actively transported from the sporulation medium into the cytoplasm of the mother cell, and then moves into the forespore by facilitated diffusion.
Synthesis of DPA by the bacterial cell begins at about the same time that Ca++ uptake begins. DPA is unique to sporulation, and in the spores of most species occurs in about 1:1 molar ratio with Ca++. It is almost always extracted from spores as the calcium-DPA chelate. DPA may have a possible role in the facilitated diffusion of calcium across the two forespore membranes. In the, sporulating cell the total DP A is confined to the forespore compartment. A model for calcium transport during sporulation proposes that:
(i)an active transport mechanism of the mother cell membrane results in influx of Ca++ from the external medium into the cytoplasm of the mother cell to a concentration level of 3-9mM, and (ii) a facilitated diffusion mechanism transports the Ca++ from the cytoplasm into the forespore compartment. Here it chelates with DPA. Continuous synthesis of DPA and chelation with Ca++ permits the forespore to continually withdraw Ca++ from the cytoplasm and yet maintain a low free Ca++ concentration in the forespore .Synthesis of the enzymes adenosine deaminase and ribosidose takes place during stage IV. The spore becomes heat resi8tant following the uptake of Ca++ and synthesis of DAP.
	The pathway for DAP synthesis is related to the pathway for the biosynthesis of the aspartate family of amino acids. In some cases the cortex is differentiated into a thinner inner layer and a thicker outer layer. The inner layer is in contact with, inner forespore membrane and is called the primordial cell wall. It develops into the cell wall of the outgrowing cell after germination. It has been seen that the inner and the outer forespore membranes have reversed polarity. Spore cortex peptidoglycan is laid down between the two membranes. In B. sphaericus the primordial cell wall develops under the control of the inner forespore membrane. The thicker outer layer of the cortex is formed from cytoplasmic precursors by enzymes of the outer fore8pore membrane. Linnet and Tipper (1976) have shown that during sporulation the enzymes required for peptidoglycan synthesis are synthesized in two periods. In the first period (stages II-III) are synthesized the enzymes required for making precursor8 of peptidoglycan. In the second period (stage IV) cortex-specific meso-diaminopimelyl ligase is made. This enzyme is confined to the mother-cell compartment. It was therefore proposed that the assembly of the cortex takes place during the second period of enzyme synthesis, while the primordial cell wall is deposited in the first period

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