Lack of a relation between deoxyribonucleic acid methylation and thymineless death in Escherichia coli

Thymine metabolism and thymineless death in prokaryotes and eukaryotes

For many years it has been known that thymine auxotrophic microorganisms undergo cell death in response to thymine starvation [thymineless death (TLD)]. This effect is unusual in that deprivation of many other nutritional requirements has a biostatic, but not lethal, effect. Studies of numerous microbes have indicated that thymine starvation has both direct and indirect effects. The direct effects involve both single- and double-strand DNA breaks. The former may be repaired effectively, but the latter lead to cell death. DNA damaged by thymine starvation is a substrate for DNA repair processes, in particular recombinational repair. Mutations in recBCD recombinational repair genes increase sensitivity to thymineless death, whereas mutations in RecF repair protein genes enhance the recovery process. This suggests that the RecF repair pathway may be critical to cell death, perhaps because it increases the occurrence of double-strand DNA breaks with unique DNA configurations at lesion sites. Indirect effects in bacteria include elimination of plasmids, loss of transforming ability, filamentation, changes in the pool sizes of various nucleotides and nucleosides and in their excretion, and phage induction. Yeast cells show effects similar to those of bacteria upon thymine starvation, although there are some unique features. The mode of action of certain anticancer drugs and antibiotics is based on the interruption of thymidylate metabolism and provides a major impetus for further studies on TLD. There are similarities between TLD of bacteria and death of eukaryotic cells. Also, bacteria have "survival" genes other than thy (thymidylate synthetase), and this raises the question of whether there is a relationship between the two. A model is presented for a molecular basis of TLD.

Methylated bases of Bacillus megaterium KM nucleic acids: comparison with Escherichia coli

Parallel studies were performed with methionineless derivatives of Escherichia coli 15 T(-) and Bacillus megaterium KM: T(-). Methylated bases are present in the total cell ribonucleic acid (RNA) of B. megaterium. The level of RNA methylation in E. coli is about 60% greater than that in B. megaterium. Although E. coli deoxyribonucleic acid (DNA) was found to contain 0.12% 5-methylcytosine (5-MC) and 0.24% 6-methylaminopurine (6-MA), methylated bases were not detected in the DNA of B. megaterium. Assuming a molecular weight of 7 x 10(9) daltons for B. megaterium DNA, it was calculated that this organism could not contain more than one molecule of 5-MC or 6-MA per genome, and that possibly no methylated bases were present. Methylated bases were also not detected in the DNA of thymine-starved B. megaterium. Crude extracts of this organism possess RNA methylase activity but no detectable DNA methylase activity.

Macromolecular synthesis and thymineless death in Mycoplasma laidlawii B

The relationships between macromolecular synthesis and viability have been studied in the pleuropneumonia-like organism Mycoplasma laidlawii B adapted to a semidefined grwoth medium. This organism exhibited an absolute growth requirement for the nucleosides uridine and thymidine, a partial requirement for guanosine and deoxyguanosine, but no requirement for adenosine, deoxyadenosine, cytosine, and deoxycytosine. Cytosine and deoxycytosine partially satisfied the requirement for uridine. Loss in viability resulted from thymidine deprivation, but not from a deficiency in other growth requirements. This phenomenon of thymineless death in a mycoplasma is similar in many respects to that reported in other bacterial systems. Chloramphenicol specifically inhibited protein synthesis and allowed deoxyribonucleic acid synthesis to proceed to only about 40% of that normally produced per generation period, while causing less inhibition of ribonucleic acid synthesis. Protein synthesis inhibition permitted thymineless death to a survival level of less than 0.5%, but ribonucleic acid synthesis inhibition resulted in a higher (10%) survival level. These results are consistent with previously noted aspects of thymineless death in Escherichia coli strains, which suggest that thymineless death is coupled to ribonucleic acid synthesis.