Mutation induction in haploid yeast after split-dose radiation exposure. II. Combination of UV-irradiation and X-rays

Synonymous point mutation of gtfB gene caused by therapeutic X-rays exposure reduced the biofilm formation and cariogenic abilities of Streptococcus mutans

BACKGROUND

The shift of oral microbiota is a critical factor of radiation caries in head and neck cancer patients after the radiotherapy. However, the direct effects of irradiation on the genome and virulence of cariogenic bacteria are poorly described. Here we investigated the genomic mutations and virulence change of Streptococcus mutans (S. mutans), the major cariogenic bacteria, exposed to the therapeutic doses of X-rays.

RESULTS

X-ray reduced the survival fraction of S. mutans and impacted its biofilm formation. We isolated a biofilm formation-deficient mutant #858 whose genome only possessed three synonymous mutations (c.2043 T > C, c.2100C > T, c.2109A > G) in gtfB gene. The "silent mutation" of c.2043 T > C in gtfB gene can cause the down-regulation of all of the gtfs genes' expression and decrease the GtfB enzyme secretion without the effect on the growth due to the codon bias. #858 and synonymous point mutation strain gtfB ^{2043 T>C}, similar to the gtfB gene null mutant Δ gtfB, can significantly decrease the extracellular polysaccharide production, biofilm formation and cariogenic capabilities both in vitro and in vivo compared with wild type.

CONCLUSION

The direct exposure of X-ray radiation can affect the genome and virulence of oral bacteria even at therapeutic doses. The synonymous mutations of genome are negligent factors for gene expression and related protein translation due to the codon usage frequency.

Optimization of lipid production by the oleaginous yeast Lipomyces starkeyi by random mutagenesis coupled to cerulenin screening

In this work we performed assays for the genetic improvement of the oleaginous yeast Lipomyces starkeyi DSM 70296 focusing on its utilization for lipid biosynthesis from renewable sources. The genetic optimization was carried out by random mutagenesis by ultraviolet irradiation and mutant selection by cerulenin, a compound displaying inhibitory effects on lipid biosynthesis. Mutants demonstrating normal growth in presence of cerulenin were considered as good candidates for further studies. Using this strategy, we selected 6 mutants for further studies, in which their productivities were evaluated by fermentation in shaken flasks and bioreactor. The evaluation of the fermentative performance of mutants was carried out using xylose as sole carbon source; the fermentation of wild-type strain was used as reference. Using this strategy it was possible to identify one mutant (termed A1) presenting a significant increase in the productivity rates of both biomass and lipid in comparison to wild-type strain. A1 mutant was further studied in bioreactor using the same fermentation parameters optimized for L. starkeyi lipid production from a mixed carbon source (xylose:glucose), as previously determined by other studies in our laboratory. A1 presented a productivity increase of 15.1% in biomass and 30.7% in lipid productivity when compared to the wild-type strain with a similar fatty acid composition, despite a slight increase (approx. 7%) on the unsaturated fraction. Our work demonstrates the feasibility of the random mutagenesis strategy coupled with mutant selection based on cerulenin screening for the genetic improvement of the oleaginous yeast L. starkeyi.