Q.66 Which one of the following metabolites is associated with bacterial stringent
response?
(A) Cyclic di–GMP (CDG)
(B) Guanosine tetraphosphate (ppGpp)
(C) Cyclic–AMP (cAMP)
(D) Cyclic–GMP (cGMP)

The metabolite associated with the bacterial stringent response is guanosine tetraphosphate (ppGpp). This alarmone triggers global metabolic reprogramming during nutrient stress in bacteria. Option (B) is correct, as confirmed by extensive research on bacterial stress mechanisms.

Correct Answer

(B) Guanosine tetraphosphate (ppGpp)
ppGpp, along with pppGpp (collectively (p)ppGpp), accumulates rapidly during amino acid, carbon, or other nutrient starvations. It binds RNA polymerase to downregulate rRNA/tRNA synthesis, halt replication, and shift resources to survival genes.​

Option Analysis

• (A) Cyclic di-GMP (CDG): Regulates biofilm formation, motility, and virulence via diguanylate cyclases and phosphodiesterases, but plays no direct role in stringent response.​

• (C) Cyclic-AMP (cAMP): Controls catabolite repression and stress growth arrest via CRP, with indirect links to (p)ppGpp but not the core stringent effector.​

• (D) Cyclic-GMP (cGMP): Rare in bacteria; involved in encystment or low-level signaling in some species, unrelated to stringent response.​

The bacterial stringent response metabolite, guanosine tetraphosphate (ppGpp), serves as the key alarmone enabling bacteria to survive nutrient starvation. This response reprograms transcription, translation, and metabolism for stress adaptation.​

ppGpp Synthesis and Mechanism

RelA synthesizes ppGpp during amino acid starvation on stalled ribosomes, while SpoT handles other stresses like carbon limitation. ppGpp binds RNA polymerase’s β’ subunit, prioritizing survival genes over growth processes like rRNA synthesis. Levels rise within minutes, inducing a “hibernation” state.​

Comparison of Metabolites

Relevance to CSIR NET

For exams like CSIR NET Life Sciences, recognize ppGpp as the hallmark of stringent response, distinguishing it from second messengers like c-di-GMP in biofilms. Mutations in relA/spoT abolish this response, highlighting its conservation across bacteria.​