Cairnsian mutagenesis inEscherichia coli: Genetic evidence for two pathways regulated bymutS andmutL genes

The phenomenon of Cairnsian mutagenesis was studied inEscherichia coli mutants bearing mutations inmutS,mutL,recA andlexA genes. It is shown that development of resistance to exogenous valine could be used as an example of Cairnsian response. Strains defective inmutS andmutL show a high frequency of Cairnsian mutagenesis to valine resistance. The response inmutS mutants is dependent upon cleavability of the LexA protein whereas that inmutL is not. The latter is independent ofrecA also. The need for LexA protein cleavage inmutS mutants can be bypassed by over-production of the RecA protein due to arecA operator constitutive mutation. Genetic evidence is presented to show that the products ofmutS andmutL genes negativelycontrol two pathways of Cairnsian mutagenesis. Cairnsian response is also elicited whenmutS ormutL strains are grown under conditions wherein a required nutrient is present in sub-optimal concentrations. Random, unselected mutagenic events are likely to occur during or after Cairnsian mutagenesis provided the cells are SOS inducible.     

Is all cyclo(His-Pro) derived from thyrotropin-releasing hormone?

Cyclo(His-Pro), or histidyl-proline diketopiperazine, is an endogenous cyclic dipeptide that is ubiquitously distributed in tissues and body fluids of both man and animals. This cyclic dipeptide is not only structurally related to thyrotropin-releasing hormone (TRH, pGlu-His-ProNH₂), but it can also arise from TRH by the action of the enzyme pyroglutamate amino-peptidase (pGlu-peptidase). The data on the distribution of TRH, cyclo(His-Pro), and pGlu-peptidase under normal and abnormal conditions are summarized and potential relationships analyzed. We conclude that all of the cyclo(His-Pro) cannot be derived from TRH. Two additional sources of cyclo(His-Pro) are suggested. It is proposed that 29,247 molecular weight TRH prohormone, prepro TRH, which contains 5 copies of TRH sequence, can be processed to yield cyclo(His-Pro). Thus, both TRH and cyclo(His-Pro) share a common precursor, prepro[TRH/Cyclo(His-Pro)].     

Structural insights into the binding mode and conformational changes of BSA induced by bixin and crocin

Bixin and crocin are natural apocarotenoids utilized as food colorants and additives in food industries worldwide. For safety assessment, it is necessary to understand the biological interaction of food colorants. In our present study, we report the interaction of two apocarotenoids with bovine serum albumin (BSA) at physiological pH using spectroscopic techniques and in silico tools. The binding constant and the mode of binding sites have been studied. The enthalpic and entropic contribution to the intermolecular binding event was analyzed and it was found that the contribution of hydrogen bonding and hydrophobic interactions was dominant. The adverse temperature dependence in the unusual static quenching is found to be a reasonable consequence of the large activation energy requirement in the binding process, which is required to overcome the fundamental block and is a direct result of the unique microstructure of the binding sites. To confirm the experimental analysis, we investigated the binding patterns using different in silico tools. A combination of molecular docking, molecular dynamics, and toxicity analysis was performed, and the obtained results revealed that both the apocarotenoids had high binding affinity with a binding energy of ?5.44 and ?5.93 kcal/mol for bixin and crocin, respectively, with no toxic effects and are in accordance with our experimental analysis. The results directly revealed the flexibility of the protein toward bixin and crocin which has a great impact on the interaction. Thus bixin and crocin can guardedly be used as food colorants in food industries.