crobial origin (Peberdy, 1988; Wayne Davies, 1991). However, heterologous proteins may show some degree of toxicity to the host and have a a major influence on the stability of heterologous protein expression. As well as restricting cell growth as biomass the toxicity will provide selective conditions for segregant cells which no longer synthesize the protein at such a high level (Goodey et al., 1987). Therefore, optimum growth conditions may be achieved by not synthesizing a heterologous protein continuously and only inducing it after the host culture has grown up in a vessel to produce sufficient biomass (Piper and Kirk, 1991). In cells of S. cerevisiae where the Gall promoter is part of the gene expression system, product formation may be induced by galactose addition to the growth medium which contains glycerol or low non-repressing levels of glucose as a carbon source.
One commercial system that has been developed is based on the ale A promoter in Aspergillus nidulans to express human interferon «2 (Wayne Davies, 1991). This can be induced by volatile chemicals, such as ethylmethyl ketone, which are added when biomass has increase to an adequate level and the growth medium contains a non-repressing carbon source or low non-repressing levels of glucose.
Methylotrophic yeasts such as Hansenula polymor-pha and Pichia pastoris may be used as alternative systems because of the presence of an alcohol oxidase
Table 4.14. Some examples of industrially important enzyme inducers
Pullulanase a-Mannosidase Penicillin acylase Proteases
Yeast mannans Phenylacetic acid Various proteins
(beet pulp, apple pomace, citrus peel) Isovaleronitrile
Aspergillus spp. Bacillus subtilis Aerobacter aerogenes Streptomyces griseus Escherichia coli Bacillus spp. Streptococcus spp. Streptomyces spp. Asperigillus spp. Mucor spp. Trichoderma uiride Aspergillus spp.
Windish and Mhatre (1965)
Wallenfels et al. (1966)
Inamine et al. (1969) Carrington (1971) Keay (1971) Aunstrup (1974)
Fogarty and Ward (1974)
Kobayashi et al. (1992)
promoter (Veale and Sudbery, 1991). During growth on methanol, which also acts as an inducer, the promoter is induced to produce about 30% of the cell protein. In the presence of glucose or ethanol it is undetectable. Expression systems have been developed with P. pastoris for tumour necrosis factor, hepatitis B surface antigen and a-galactosidase. Hepatitis B surface antigen and other heterologous proteins can also be expressed by //. polymorpha.
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