Translation of adenylate cyclase mRNA

A transcription profile of E. coli K12 (strain MG1655) indicated that adenylate cyclase mRNA is present in cells grown in specific conditions [Proc Natl Acad Sci U S A].  The mRNA signal intensity was reported to be low and adenylate cyclase was not detected most likely because of the detection limit of 100 copies per cell.  Previous data have indicated that there are only 15 molecules of adenylate cyclase in a wild type strain [J Biol Chem].  However, because other proteins with a low mRNA signal intensity were detected, the study by R. Corbin, O. Paliy and coworkers may emphasize the inefficiency of translation of adenylate cyclase mRNA.

The adenylate cyclase mRNA initiation codon is the rare triplet UUG [J Mol Biol] which serves as initiator for 3% of E. coli K12 proteins [Science Magazine].  Replacement of UUG by the more common AUG triplet caused lethality in Escherichia coli due to an increase of cyaA expression [Proc Natl Acad Sci U S A], a finding in agreement with the observation that UUG is less efficient than AUG in initiating translation [Mol Microbiol].

Polyamines (putrescine, spermidine and spermine) were shown to improve the translation of the cyaA mRNA by stimulating the initiation complex formation.  A moderate 2.5-fold increase in adenylate cyclase synthesis was observed in the presence of putrescine resulting in a three-fold increase in the cAMP level.  This increase occurred only in a polyamine-requiring strain and resulted in an increase of RNA polymerase sigma 28 synthesis via the CAP-cAMP complex [J Biol Chem].

The putrescine content of Escherichia coli cells is dependant on the osmotic strength of the medium.  A sudden increase in osmolarity results in a rapid excretion of cellular putrescine [J Biol Chem].  If, in agreement with the data described above, a decrease in cellular putrescine correlates with a decrease in the cAMP level, the transcription of RpoS (a global stress response regulator) may be enhanced upon osmotic shock, as it has been shown that CAP-cAMP is a negative regulator of rpoS transcription [Genes Dev].  Considering that RpoS is involved in osmotic regulation [J Bacteriol], the effect of putrescine on adenylate cyclase synthesis may be physiologically relevant, under specific conditions.

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