Suppression of gene expression by homologous transgenes

When a wild-type strain of Neurospora crassa is transformed with different portions of the carotenogenic albino 1 or albino 3 genes, up to 30-35% of the transformants show an albino phenotype. The albino transformants presented a variety of phenotypes ranging from white or yellow to dark yellow colour. The ectopically integrated sequences provoke a severe impairment of the expression of the endogenous al-1 or al-3 genes. This phenomenon, that has been termed quelling, is found to be spontaneously and progressively reversible. In fact, all of the albino transformants have an unstable phenotype and revert progressively to wild type or intermediate phenotypes over a prolonged culturing time. The phenotypic reversion is characterised by a progressive release of the transcriptional inhibition and seems to correlate with the reduction of the number of the ectopic integrated sequences. However, there is no strict correlation between the copy number of the ectopic sequences and the intensity of quelling, as indicated by the existence of albino transformants containing only 1-2 ectopic sequences. The nature of the molecular events determining the onset of quelling is unclear, in any event, these are likely to involve some kind of interaction between the resident genes and ectopically integrated exogenous sequences. Recent evidences on a possible mechanism are presented.