III-4. Radiation Chemistry & Biology

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Eﬀect of irradiation ions and doses on the survival rate of yeast
N. Lei,∗1 T. Abe,∗1 and H. Ichida∗1
Yeast has been playing an important role in many human activities, such as winemaking, baking, and brewing
for a long period.1) The budding yeast, Saccharomyces
cerevisiae, has been considered as the most fundamental eukaryotic model organism. Its small genome size(around 12 Mb), fast growth character, and DNA repair system that is similar to that of plants and animals
make it a useful model for gene function and mutation
analyses.2,3)
Heavy ion beams cause mutations in the genome, suchas deletions, insertions, and base substitutions. Therefore, it has been used as a physical mutagen and to develop new cultivars in the last two decades. Carbon ionshave been used frequently because of the high mutation
rates and long penetration range, and heavier ions, suchas argon and iron, are more suitable for microbial mutagenesis, as they induce larger deletions and insertions,and are more stable than base substitutions.4)
In the present study, we investigated the dose-survival
relationship of a diploid baker’s yeast strain Hi1060(MATa/α his3 Δ1 leu2 Δ0 ura3 Δ0 ). It is geneticallyequivalent to BY4743 and is established by a cross between BY4741 and BY4742; It is more than three times
the size of single-colony isolations on a selective medium.
The strain and its parents are all derivatives of thewidely used and genome-sequenced laboratory strain
S288C. This strain is fully functional for DNA repair
by homologous recombination and non-homologous endjoining.
For heavy-ion beam irradiations, Hi1060 was grownin a liquid YPD medium, harvested by centrifugation at3000 × g, and washed three times with 10 mM MgSO4 .
The cell concentration was adjusted to 1 OD600 unit permL, which is equivalent ∼107 cells per mL, with 10 mM
MgSO4 , and was kept at 4◦ C before and after irradiation. We irradiated 12 C6+ (135 MeV/nucleon, LET:
23 keV/μm; 0–1000 Gy), 40 Ar18+ (160 MeV/nucleon,
189 keV/μm; 0–800 Gy), and 56 Fe24+ (90 MeV/nucleon,
647 keV/μm; 0–800 Gy). After irradiation, the cells
were diluted to diﬀerent concentrations and spread on
YPD plates, and the survival rates were calculated asfollows:
Survival rate (%) = num. of the colonies in eachdose/non-irradiated control × 100
The determined survival rates and their ﬁtting resultsby exponential functions are shown in Fig. 1. The results showed that carbon-, argon- and iron-ion irradiations all decreased the survived cell ratio in a dosedependent manner. The survival curves were well represented by exponential functions with determination coeﬃcients (R2 ) of 0.9793, 0.7691, and 0.9509 in carbon,argon, and iron ions, respectively. Ar showed unexpectedly higher and lower survival rates compared with the∗1Fig. 1. Dose-survival responses of S. cerevisiae Hi1060(equivalent to BY4743) cells after irradiation by carbon,
argon, and iron ions with diﬀerent doses. The error bars
represent standard deviation in technical replicates.

other two ions. We are interested in further investigating
Ar irradiations soon.
The lethal eﬀects were more obvious in heavier argon
and iron ions than the lighter carbon ions. This is consistent with the previous observations that the relative
biological eﬀect of a radiation is higher in heavier ionsthan the lighter ones. This may reﬂect that heavier ions,which have higher LETs, induce more deleterious typesof mutations to the genome. The doses for 10% survival(D10 ) were 600 Gy for carbon- and 400 Gy for argonand iron-ions, respectively. These D10 values were in acomparable range to a previous yeast breeding project,
where 400 Gy irradiations with carbon and iron ionswere used to select high aromatic compound-producing
sake yeasts.5) Currently, we have been preparing to perform whole-genome sequencing of these irradiated yeast
cells. This will quantitatively describe the mutagenic
eﬀect of diﬀerent ions on yeast genomes. At the sametime, it would help to understand the relationship between irradiation conditions, survival rate, and the resulting genomic mutations in a base-pair resolution in agenome-wide manner.
References
1) R. K. Mortimer, Genome Res. 10, 403 (2000).
2) M. J. Harsch et al., FEMS Yeast Res. 10, 72 (2009).
3) H. Mewes et al., Nature 387, 7 (1997).
4) H. Ichida et al., Mutat. Res. 639, 101 (2008).
5) M. Yokobori et al., RIKEN Accel. Prog. Rep. 45, xxiii(2011).

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