User:Adarsh.chavakula/Draft

Colanic Acid is a polymer of high molecular weight containing glucose, galactose, fucose and glucaronic acid, which is modified with acetyl and pyruvyl groups. It is an exopolysaccharide acid secreted by E.Coli when exposed to hostile environments. Under conditions of stress, such as extreme pH, extreme temperature, dryness and osmotic shock, this protein forms a capsule which surrounds the cell surface. . It is synthesized by the expression of the cps (capsular polysaccharide synthesis) genes, the transcription of which is regulated by the expression of the rcs (regulator of capsule synthesis) genes.

Structure and Function
Synthesis of colonic acid requires the presence of two regulatory proteins – RcsA and RcsB, synthesized by the expression of the genes rcsA and rcsB respectively. rcsA lies on the min 43 of the E.Coli map and rcsB lies on the min 47. A third protein, RcsC, synthesized by rcsC, is also essential for capsule regulation



Degradation of RcsA by Lon protease
RcsA is stabilized in the absence of Lon protease (synthesized by the expression of lon gene) and hence is essential for the expression of cps genes. Lon carries out hydrolysis of RcsA in the presence of ATP

Activation of RcsB by RcsC
RcsC and RcsB act as a two-component regulatory pair. RcsC acts as a constitutively active sensor kinase and activates RcsB by means of phosphorylation.

Interaction between RcsA and RcsB
RcsA is significantly stabilized by RcsB and is absolutely essential for RcsA function. However requirement of RcsA can be partially bypassed by excess RcsB or excess constitutively active RcsC. The cps genes are expressed most strongly when all three are present.

Eukaryotic Homologue
The capsular protein polysaccharide is involved in capsule formation, which is one of the strictly restricted capabilities of prokaryotes to survive extreme conditions. Such a mechanism is rare in eukaryotes. However, a pathogenic fungus called the Cryptococcus neoformans. C.neoformans is rare in a sense due to the fact that it is the only fungus that is pathogenic to humans as well as able to produce a true polysaccharide capsule outside its cell wall. Tremella mesenterica, a close phylogenetic relative, possesses a similar capsule but is not pathogenic. Some of the other less renowned fungi that have capsule-like structures include Malassezia furfur, Rhinosporidium seeberi, Trichosporon beigelii, Blastocystis hominis, and Sporothrix schenckii. But they have not yet been understood fully.