Ribosome ppGpp

The ribosome-associated ppGpp synthetase (RelA) is required for antibiotic production under the conditions of nitrogen limitation in S. coelicolor A3 and for cephamycin C production in S. clavuligerus. Deletion of the relA in S. coelicolor A3 results in the loss of production of the antibiotics actinorhodin (Act) and undecylprodigiosin (Red) and... 

Gralla JD, Escherichia coli ribosomal RNA transcription Regulatory roles for ppGpp, NTPs, architectural proteins and a polymerase-binding protein. Mol Microbiol 55 973-977, 2005. 

Schematic diagram of ppGpp synthesis and the hypothesized mechanism for its action. ppGpp is synthesized on the ribosome when there is a peptidyl-tRNA on the P site of the ribosome and uncharged tRNA on the A site. The ppGpp probably inhibits rRNA synthesis by complexing with the RNA polymerase.

The observations on ppGpp s role in rRNA synthesis show that this nucleotide is an important control factor regulating rRNA synthesis, but it does not eliminate the possibility that other factors also affect the level of rRNA. In vivo and in vitro evidence indicates that the inhibitory effect of ppGpp on transcription extends to most tRNA and riboso-mal protein genes. Ribosomal protein gene expression also appears to be regulated at the translational level. 

Ochi K. A relaxed (rel) mutant of Streptomyces coelkolor A3(2) with a missing ribosomal protein lacks the ability to accumulate ppGpp, A factor and prodigiosin. J Gen Microbiol 1990 136 2405-2412. 

The essential elements for controlled transcription in a purified system are lac DNA, RNA polymerase (holoenzyme), cyclic AMP binding protein, cyclic AMP, lac repressor and inducer (Crombrugghe et al., 1971b Eron et al., 1971). However, other experiments give some doubt as to the completeness of the control system ppGpp has been reported to stimulate the transcription of the lac operon in the presence of ribosomal wash (Crombrugghe et al.. 

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