Giuseppe Macino

Neurospora crassa is a central organism in the history of twentieth-century genetics, biochemistry and molecular biology. Here, we report a high-quality draft sequence of the N. crassa genome. The approximately 40-megabase genome encodes about 10,000 protein-coding genes--more than twice as many as in the fission yeast Schizosaccharomyces pombe and only about 25% fewer than in the fruitfly Drosophila melanogaster. Analysis of the gene set yields insights into unexpected aspects of Neurospora biology including the identification of genes potentially associated with red light photobiology, genes implicated in secondary metabolism, and important differences in Ca2+ signalling as compared with plants and animals. Neurospora possesses the widest array of genome defence mechanisms known for any eukaryotic organism, including a process unique to fungi called repeat-induced point mutation (RIP). Genome analysis suggests that RIP has had a profound impact on genome evolution, greatly slowing the creation of new genes through genomic duplication and resulting in a genome with an unusually low proportion of closely related genes.

Up to 36% of Neurospora crassa transformants showing an albino phenotype were recovered by transforming a wild-type strain with different portions of the carotenogenic albino-3 (al-3) and albino-1 (al-1) genes. The presence of the exogenous sequences (which were randomly integrated in ectopic locations) provoked a severe impairment in the expression of the endogenous al-1 or al-3 genes. This phenomenon, which we have termed 'quelling', was found to be spontaneously and progressively reversible, leading to wild-type or intermediate phenotypes. The phenotypic reversion is characterized by a progressive release of the transcriptional inhibition and seems to correlate with a reduction of the number of the ectopic integrated sequences. Moreover, quelling appears to be monodirectional, as, once relieved, it cannot take place again, despite the continuing presence of some of the ectopic sequences in the genome.

Although it is known that the human genome contains hundreds of microRNA (miRNA) genes and that each miRNA can regulate a large number of mRNA targets, the overall effect of miRNAs on mRNA tissue profiles has not been systematically elucidated. Here, we show that predicted human mRNA targets of several highly tissue-specific miRNAs are typically expressed in the same tissue as the miRNA but at significantly lower levels than in tissues where the miRNA is not present. Conversely, highly expressed genes are often enriched in mRNAs that do not have the recognition motifs for the miRNAs expressed in these tissues. Together, our data support the hypothesis that miRNA expression broadly contributes to tissue specificity of mRNA expression in many human tissues. Based on these insights, we apply a computational tool to directly correlate 3' UTR motifs with changes in mRNA levels upon miRNA overexpression or knockdown. We show that this tool can identify functionally important 3' UTR motifs without cross-species comparison.