Oxidized guanine (8-oxo-7,8-dihydroguanine; 8-oxo-G) is a potent mutagen because of its ambiguous pairing with cytosine and adenine. The Escherichia coli MutT protein specifically hydrolyzes both 8-oxo-deoxyguanosine triphosphate (8-oxo-dGTP) and 8-oxo-guanosine triphosphate (8-oxo-rGTP), which are otherwise incorporated in DNA and RNA opposite template A. In vivo, this cleaning of the nucleotide pools decreases both DNA replication and transcription errors. The effect of mutT mutation on transcription fidelity was shown to depend on oxidative metabolism. Such control of transcriptional fidelity by the ubiquitous MutT function has implications for evolution of RNA-based life, phenotypic expression, adaptive mutagenesis, and functional maintenance of nondividing cells.

Various environmental insults result in irreversible damage to proteins and protein complexes. To cope, cells have evolved dedicated protein quality control mechanisms involving molecular chaperones and proteases. Here, we provide both genetic and biochemical evidence that the Lon protease and the SecB and DnaJ/Hsp40 chaperones are involved in the quality control of presecretory proteins in Escherichia coli. We showed that mutations in the lon gene alleviate the cold-sensitive phenotype of a secB mutant. Such suppression was not observed with either clpP or clpQ protease mutants. In comparison to the respective single mutants, the double secB lon mutant strongly accumulates aggregates of SecB substrates at physiological temperatures, suggesting that the chaperone and the protease share substrates. These observations were extended in vitro by showing that the main substrates identified in secB lon aggregates, namely proOmpF and proOmpC, are highly sensitive to specific degradation by Lon. In contrast, both substrates are significantly protected from Lon degradation by SecB. Interestingly, the chaperone DnaJ by itself protects substrates better from Lon degradation than SecB or the complete DnaK/DnaJ/GrpE chaperone machinery. In agreement with this finding, a DnaJ mutant protein that does not functionally interact in vivo with DnaK efficiently suppresses the SecB cold-sensitive phenotype, highlighting the role of DnaJ in assisting presecretory proteins. Taken together, our data suggest that when the Sec secretion pathway is compromised, a pool of presecretory proteins is transiently maintained in a translocation-competent state and, thus, protected from Lon degradation by either the SecB or DnaJ chaperones.Molecular chaperones comprise a large group of highly conserved proteins that aid protein folding, transit across biological membranes, and quality control as well as assembly and disassembly of protein complexes (1-3). Molecular chaperones act by repeated binding and release of aggregation-sensitive polypeptide segments normally found buried in the final folded structure of a substrate. Although predominantly found during de novo protein synthesis or after a denaturing stress leading to protein misfolding or aggregation, such aggregation-prone segments can also be transiently exposed in native multiprotein complexes. In this case molecular chaperone binding coordinates subtle conformational changes affecting biological functions (4, 5).In the bacterium Escherichia coli, intracellular protein folding is mainly orchestrated by the chaperones Trigger Factor (TF), 2 DnaK/Hsp70, and GroEL/Hsp60. The chaperone TF binds near the ribosomal polypeptide exit with a 1:1 stoichiometry, forming a protective "cradle" for emerging nascent chains. Because of its advantageous localization, TF may interact cotranslationally with most nascent polypeptides (6, 7). TF cycles on and off the ribosome, and both ribosome-bound or released polypeptide chains can stay post-translationally bound to ribosome-free TF for a prolonged period ...

Background:De novo protein folding is assisted by essential networks of molecular chaperones. Results: Overproduction of Hsp33 controls EF-Tu stability thus allowing bacterial growth without trigger factor and DnaK. Conclusion: Slowing down translation limits protein aggregation and enables bacterial survival in the absence of major chaperones. Significance: Deciphering networks of chaperones is crucial for understanding how cells respond to severe protein aggregation.