Modification of UV-induced mutation frequency and cell survival of Escherichia coli B/r WP2 trpE65 by treatment before irradiation

Genetic diversity based on multivariate techniques in elephant grass genotypes for bioenergy

The use of biomass for energy production constitutes a promising strategy that warrants the search for new sources of biomass. Elephant grass has been gaining notoriety due to its high dry matter yield and rapid growth. The present study was carried out to quantify the genetic divergence of nine elephant grass half-sib families in order to identify genotypes with greater genetic divergence and productive potential for hybridization, using the hierarchical clustering methodology based on principal components. Half-sib families were generated using genotypes from the Active Germplasm Bank of Elephant Grass. The experiment was laid out in a randomized-block design with nine half-sib families, three replicates, and eight plants per plot. A total of 216 genotypes of elephant grass were evaluated. Principal component (PC), biplot, and hierarchical clustering analyses for diversity estimation were conducted using R software. The first two PCs of biplot analysis accounted for 64% of the cumulative variation. Dry matter yield was the most important trait for genotype discrimination (0.89), followed by plant height (0.67) and stem diameter (0.61) in PC1. In this analysis, the distances between accessions were considered and there were no family links, which indicates the existence of wide variability within the evaluated families, since genotypes belonging to the same family were not grouped together, but rather distributed into different groups. Crosses between genotypes of group three and genotypes of groups one and two are recommended for the development of high-yielding genotypes when aiming at energy production.

Comparative mutagenesis of Escherichia coli strains with different repair deficiencies irradiated with 222-nm and 254-nm ultraviolet light

Photoinactivation and reversion to tryptophan prototrophy were studied in four Escherichia coli strains with different repair deficiencies. Cells were irradiated with 222-nm wavelength UV emitted by an excimer lamp and with 254-nm wavelength UV emitted by a low-pressure mercury lamp. Strain DSM 9494 (trp(-)uvrA(+)) turned out to be most resistant while the strain DSM 9495 (trp(-)uvrA(-)), which is defective in nucleotide-excision repair (NER) was most sensitive to both wavelengths. UV-fluence rates for a respective inactivation were twice as high for 222-nm wavelength UV than for 254-nm UV. No clear difference in efficiency of inactivation could be observed between the two wavelengths in strains DSM 9496 (trp(-)uvrA(+) pKM101) and DSM 9497 (trp(-)uvrA(-) pKM101). In general, more revertants were induced by 254-nm wavelength UV, which corroborates the hypothesis that a higher amount of DNA damage was induced by this wavelength than by 222-nm UV, except for DSM 9497 where no clear difference could be observed regarding the number of revertants induced by both wavelengths. This strain DSM 9497 has a high sensitivity to certain oxidative mutagens compared with other strains, which is indicative of formation of reactive oxygen species during irradiation with 222-nm wavelength UV.

DNA-replication recovery inhibition and subsequent reinitiation in UV-radiation-damaged E. coli: a strategy for survival

Using the incorporation of [14C]thymine to measure DNA accumulation, it was shown that exposure of the B/r strain of Escherichia coli to 10 J/m2 of ultraviolet radiation (UV) inhibits replication for about 20 min, but then resumption of replication occurs. Pulse-labelling with [3H]thymidine after exposure of the WT strain to this fluence confirmed the transient inhibition and recovery of DNA replication. After recovery, the rate of accumulation of DNA in the culture increases, to exceed that of the exponentially growing culture, so that eventually the amount of DNA almost equals that of the unirradiated culture. After a higher fluence (20 J/m2), an inhibition of replication recovery was revealed. This fluence delays the reinitiation of DNA accumulation in the culture, measured by [14C]thymine incorporation, for 25 min more, in addition to the 20-min recovery period. This finding was confirmed with pulse-labelling studies, which revealed that the higher exposure represses the rates of replication for 45 min before replication at the normal rate reinitiates in the culture. It was proposed that the inhibition of recovery revealed by these investigations is effected by the UV-induction of an active DNA-replication recovery-inhibition process. With the uvrA strain, rate studies revealed that 1.5 J/m2 of UV (a reduced fluence necessary because of the greater sensitivity of the strain) induces a transient inhibition of DNA replication, with considerable recovery following. Exposure to 3.0 J/m2 induces the transient inhibition of replication, followed by massive recovery inhibition after 20 min of incubation. With uvrA recA, both the lower and the higher fluence resulted in an immediate block of replication with no recovery, confirming the recA gene dependency of the recovery process. The decrease in rate of replication comparable to that seen in the uvrA strain after 20 min, and taken as evidence of the function of the recovery-inhibition process, was not seen. The evidence supports the concept that a process somehow triggered by higher UV fluences functions to repress replication temporarily, presumably allowing time for repair processes to take place before replication overruns closely linked pyrimidine dimers on opposite strands to create lethal lesions.

Evidence that ultraviolet light-induced DNA replication death of recA bacteria is prevented by protein synthesis in repair-proficient bacteria

The ultraviolet light (UV) survival curve of Escherichia coli WP10 recA trp is almost biphasic, with a greatly reduced shoulder but demonstrating a transition to a decreased slope with increasing fluences, indicating the presence in the culture of a low frequency of resistant cells. Treatment of the culture with chloramphenicol before UV exposure brought almost all of the cells to a high degree of UV resistance, by bringing them to the end of their DNA replication cycle. The survival curves of the repair-proficient E. coli WP2 trp showed a similar pattern with chloramphenicol treatment or tryptophan starvation before UV exposure, but only if protein synthesis were blocked by chloramphenicol for 60 min after UV exposure. The results suggest that when recA/lexA-regulon induction is prevented, either by the recA mutation or by inhibition of protein synthesis after UV exposure, death occurs unless the cells are in the resistant state characteristic of bacteria at the end of their DNA replication cycle. With repair-proficient bacteria treated before UV exposure with chloramphenicol, when protein synthesis is not blocked after UV exposure, a marked expansion of the shoulder occurs because of the function of another resistance-conferring mechanism. This mechanism also depends on the recA+ gene since expansion of the shoulder does not occur in recA bacteria when protein synthesis is inhibited before UV exposure.

Modification of survival after ultraviolet light exposure in a wild-type and a polA strain of Escherichia coli B/r by preirradiation treatment with chloramphenicol or rifampin

The shoulder of the UV fluence-survival curve of exponentially growing Escherichia coli B/r WP2 trpE65 was expanded by chloramphenicol pretreatment and an exponential segment with intermediate slope appeared between the shoulder and the final exponential segment. These changes were dependent on DNA replication. The transitions with UV exposure to increased slopes were ascribed to UV inactivation of qualitatively different repair systems, each dependent upon the accumulation in each bacterium of multiple DNA-containing redundant repair components, which must be inactivated before the respective transitions to decreased resistance occur. Rifampin, which blocks DNA-dependent RNA polymerase function, limited drastically expansion of the shoulder and development of the intermediate exponential slope. Bacteria defective in DNA polymerase I (polA) showed only a slight expansion of the shoulder with pretreatment with chloramphenicol. Since certain bacterial plasmids require RNA primer formation for initiation of replication and are not maintained in a polA strain, it is proposed that the chloramphenicol-promoted increase in resistance depends on the formation of multiple numbers of specific resistance episomes (called repairons in view of their role in DNA repair).

Chloramphenicol-promoted increase in resistance to UV damage in Escherichia coli B/r WP2 trpE65: development of the capacity for successful repair of otherwise mutagenic or lethal lesions in DNA

The ultraviolet radiation survival curve of exponentially growing cultures of Escherichia coli B/r WP2 trpE65 was modified by a short period (20 min) of chloramphenicol treatment before UV exposure, which produced an extended exponential section of intermediate slope between the shoulder and the final exponential slope. More prolonged incubation with chloramphenicol (up to 90 min) resulted in little further extension of the intermediate exponential slope, but caused a progressive expansion of the shoulder region. With each period of chloramphenicol pretreatment, a major surge of mutation to tryptophan independence always occurred after that UV fluence promoting the transition from the shoulder to the intermediate exponential slope of the survival curve, and another major surge occurred after that fluence promoting the transition from the intermediate exponential slope to the final exponential slope. A minor surge of mutation occurred after low fluences. The 3 surges in mutation and the increased slopes of the survival curve are ascribed to UV-inactivation of 3 qualitatively different DNA-repair systems, each with differentially increased resistances to UV caused by pretreatment by chloramphenicol.