TRANSGENE SILENCING OF THE AL-1 GENE IN VEGETATIVE CELLS OF NEUROSPORA IS MEDIATED BY A CYTOPLASMIC EFFECTOR AND DOES NOT DEPEND ON DNA-DNAINTERACTIONS OR DNA METHYLATION

The molecular mechanisms involved in transgene-induced gene silencing ('quelling') in Neurospora crassa were investigated using the caroteno id biosynthetic gene albino-1 (al-1) as a visual marker, Deletion deri vatives of the al-1 gene showed that a transgene must contain at least similar to 132 bp of sequences homologous to the transcribed region o f the native gene in order to induce quelling, Transgenes containing o nly al-1 promoter sequences do not cause quelling, Specific sequences are not required for gene silencing, as different regions of the al-1 gene produced quelling, A mutant defective in cytosine methylation (di m-2) exhibited normal frequencies and degrees of silencing, indicating that cytosine methylation is not responsible for quelling, despite th e fact that methylation of transgene sequences frequently is correlate d with silencing. Silencing was shown to be a dominant trait, operativ e in heterokaryotic strains containing a mixture of transgenic and non -transgenic nuclei. This result indicates that a diffusable, trans-act ing molecule is involved in quelling, A transgene-derived, sense RNA w as detected in quelled strains and was found to be absent in their rev ertants. These data are consistent with a model in which an RNA-DNA or RNA-RNA interaction is involved in transgene-induced gene silencing i n Neurospora.