- Guanosine pentaphosphate
(p)ppGpp, guanosine pentaphosphate or tetraphosphate is an
alarmone which is involved in thestringent response inbacteria , causing theinhibition ofRNA synthesis when there is a shortage ofamino acid s present. This causes translation to decrease and theamino acid s present are therefore conserved. (p)ppGpp is an effector molecule which is produced as a result of amino acid starvation within a bacterial cell.(p)ppGpp is created via pppGppp
synthase , also known as "Rel A", and is converted from pppGpp toppGpp via pppGpp phosphohydrolase. Rel A is associated with about every one in two hundredribosome s and it becomes activated when an unchargedTransfer RNA (tRNA) molecule enters the A site of the ribosome, due to the shortage of amino acid required by the tRNA. If amutant bacterium is "relA"- it is said to be relaxed and no regulation of RNA production due to amino acid absence is seen.Targets of (p)ppGpp include
rRNA operon s, of which there are 7, all of which have 2promoter s. when (p)ppGpp associates with the promoter it stops theRNA polymerase enzyme from being able to bind and initiate transcription. It is thought that (p)ppGpp may affect the stability of the open complex of the RNA polymerase and DNA or it may affect promoter clearance. Its presence also leads to an increase in pausing duringelongation and it competes withnucleoside triphosphate substrate s.When the amino acid balance in the cell is restored, (p)ppGpp is hydrolysed by
SpoT .References
*Condon et al. (1995) Microbiol Rev 59, 623
*Artsimovitch et al. (2004) Cell 117, 299
*Magnusson et al. (2005) Trends in Microbiology 13, 236
Wikimedia Foundation. 2010.