Quelling: transgene-induced gene silencing in Neurospora crassa

Abstract

During recent years evidence of the capability of some fungi and plants to recognise and modify duplicated sequences of integrated transforming DNA have been provided. In Neurospora crassa it has been shown that duplicated genes are irreversibly inactivated by a mechanism called RIP (Repeat-Induced Point mutation). Duplicated sequences are extensively modified by point mutation, namely cytosine-timine transitions (Selker et al, 1987). N. crassa is also able to inactivate duplicated sequences introduced by transformation through a phenomenon termed quelling (Romano and Macino, 1992). Unlike RIP, quelling occurs in the vegetative phase of the Neurospora life cycle and consists of a transient inactivation of the gene expression as consequence of transformation with homologous sequences. A substantial difference between RIP and quelling is that the former takes place in dikaryotic cells resulting from fertilisation, and the latter in coeneocytic cells where the exogenous DNA is introduced by transformation. Gene inactivation by quelling is not the result of mutagenesis as in RIP, instead it is the result of the suppression of gene expression, through a reduction of the steady-state mRNA level of the duplicated gene. Quelling is reversible and the function of the inactivated gene may be restored after prolonged culturing time.