Stringent response
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The stringent response occurs in all prokaryotes and some plants in reaction to amino-acid starvation or carbon starvation. The stringent response is capable of modulating up to 1/3 of all genes in a cell. It causes the cell to divert resources away from growth and division and toward amino acid synthesis. (p)ppGpp production is mediated by the ribosome parts: RelA and L11A when there are no charged tRNA's available to bind to the ribosome. RelA converts GTP and ATP into pppGpp by adding the pyrophosphate from ATP onto the 3' carbon of the ribose in GTP releasing AMP.
pppGpp is converted to ppGpp by the gpp gene product, releasing Pi. ppGpp is converted to GDP by the spoT gene product, releasing pyrophosphate (PPi). GDP is converted to GTP by the ndk gene product. NTP provides the Pi, and is converted to NDP.
The cycle repeats when GTP is reacted with ATP by the RelA gene.
(p)ppGpp inhibits other energy-consuming cell process, such as replication and transcription.
ATP ----> AMP
GTP ----> ppGppp
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