Since the novel yeast strain is obtained by genetic modification, it was assessed to establish that it presented no greater hazard to production and bakery workers, to the environment, or to consumers.
Since the novel yeast strain is obtained by genetic modification, it was assessed to establish that
it presented no greater hazard to production and bakery workers, to the environment, or to consumers
of food containing the yeast than did the unmodified strain (concept of substantial equivalence). The
evaluation of the safety of the modified yeast to workers and to the environment is not described in
this study.
The evaluation of the novel strain as a novel food took into account that consumption of both live
and dead cells could occur. Specific aspects considered included:
a) Characteristics of the host and donor organisms
The host organism is a well-characterised strain of the non-pathogenic species S. cerevisiae that
is used extensively in the industrial production of baker’s yeast for leavening sweet dough. The
natural inducible promoters for the maltase and maltose permease genes were removed and replaced
by strong, constitutive promoters from the same strain of baker’s yeast.
b) Genetic modification procedure
The donor DNA was taken entirely from S. cerevisiae, apart from small pieces of synthetic,
non-coding DNA used as linker sequences. It consisted of genes coding for the enzymes maltase and
maltose permease, together with two well-characterised, strong, constitutive promoters.
At each stage of the transformation procedure, the construct was cloned in E. coli and restriction
enzyme digestion and/or sequence analysis were used to confirm that the sequences were as predicted.
A schematic presentation of the insert showing its location on the chromosome was available.
This confirmed that no untoward effects were likely from the insertion.
The transformation procedure was designed to ensure that the construct was integrated into the
chromosome and was devoid of any heterologous DNA. Antibiotic resistance markers used to
facilitate the transformation procedure were removed, and no prokaryotic sequences remain in the
genetically modified yeast.
c) Genetically modified organism
Hybridisation patterns of DNA from the genetically modified strain were unchanged after
100 generations of vegetative growth, indicating that the strain was as stable as conventional strains
(concept of substantial equivalence).42
All heterologous prokaryotic DNA had been eliminated during the transformation procedure, and
the Southern-blot experiments had demonstrated the stability of the insert. Transfer of DNA from the
genetically modified baker’s yeast to other organisms is unlikely. It is known that on cell death in
unmodified strains of S. cerevisiae, autolysis of the cell contents occurs before the cell wall is
destroyed and no free DNA, which could be taken up by other organisms, is released. Mating or
normal exchange of DNA does not occur between S. cerevisiae and any known fungal or bacterial
pathogens, and no known DNA viruses are harboured by S. cerevisiae which might transfer DNA to
other organisms. This indicates that the risk of DNA transfer from the genetically modified baker’s
yeast would be no different from that of its transfer from the unmodified parent strain (concept of
substantial equivalence).
Production of toxic metabolites by the genetically modified strain is unlikely for several reasons:
the host organism is non-pathogenic; the donor DNA was obtained from the same strain as the host;
only homologous, constitutive promoters were rear-ranged; and the initial activities of the genes
controlling maltase and maltose permease only are affected. The biochemical reactions occurring
during the leavening process are the same in the genetically modified strain as in the unmodified
strain since both produce maltase and maltose permease, though less efficiently in the unmodified
strain (concept of substantial equivalence)
According to dbtbiosafety