Thymineless death is inhibited by CsrA in Escherichia coli lacking the SOS response

UvrA expression of Lactococcus lactis NZ9000 improve multiple stresses tolerance and fermentation of lactic acid against salt stress

Lactococcus lactis is subjected to several stressful conditions during industrial fermentation including oxidation, heating and cooling, acid, high osmolarity/dehydration and starvation. DNA lesion is a major cause of genetic instability in L. lactis that usually occurs at a low frequency, but it is greatly enhanced by environmental stresses. DNA damages produced by these environmental stresses are thought to induce DNA double-strand breaks, leading to illegitimate recombination. Nucleotide excision repair (NER) protein UvrA suppresses multiple stresses-induced illegitimate recombination. UvrA protein can survive a coincident condition of environmental harsh conditions, multiple stress factors supposedly encountered in the host and inducing UvrA in L. lactis. In this study the expression of UvrA and growth performance and viability of control strain L. lactisVector and recombinant strain L. lactisUvrA under multiple stress conditions were determined. The recombinants strain had 30.70 and 52.67% higher growth performances when subjected to acidic and osmotic stresses conditions. In addition, the L. lactisUvrA strain showed 1.85-, 1.65-, and 2.40-fold higher biomass, lactate production, and lactate productivity, compared with the corresponding values for L. lactisVector strain during the osmotic stress. Results demonstrated NER system is involved in adaptation to various stress conditions and suggested that cells with a compromised UvrA as DNA repair system have an enhanced protection behavior in L. lactis NZ9000 against DNA damage.

A balanced perspective on unbalanced growth and thymineless death

The early history of the esoteric phenomenon of thymineless death (TLD) is recounted, from the pioneering discovery by Seymour Cohen and Hazel Barner, through my graduate studies at Yale and postdoctoral research in Copenhagen. My principal contribution was the discovery that restricted synthesis of protein and RNA permits cultures of Escherichia coli to complete their DNA replication cycles without initiating new ones, and that cells held in this physiological state are immune to the lethality of thymine deprivation; unbalanced growth is not the fundamental cause of TLD. The successful synchronization of the DNA replication cycle contributed to formulation of the replicon concept. Studies at Stanford revealed a specific requirement for transcription and led to the discovery of a TLD-resistant mutant in a new gene, termed recQ, with important homologs in humans and most other organisms. The lessons learned from research on TLD underscore the value of basic research in bacterial systems that can have profound implications for human health.

Thymineless death, at the origin

Thymineless death (TLD) in bacteria has been a focus of research for decades. Nevertheless, the advances in the last 5 years, with Escherichia coli as the model organism, have been outstanding. Independent research groups have presented compelling results that establish that the initiation of chromosome replication under thymine starvation is a key element in the scenario of TLD. Here we review the experimental results linking the initiation of replication to the lethality under thymine starvation and the proposed mechanisms by which TLD occurs. The concept of this relationship was ‘in the air,’ but approaches were not sufficiently developed to demonstrate the crucial role of DNA initiation in TLD. Genome-wide marker frequency analysis and Two Dimensional agarose gel electrophoresis have been critical methods employed to reveal that initiation events and the degradation of the oriC region occur during thymine starvation. The relationships between these events and TLD have established them to be the main underlying causes of the lethality under thymine starvation. Furthermore, we summarize additional important findings from the study of different mutant strains, which support the idea that the initiation of chromosomal replication and TLD are connected.