Silencing of the Gene hph in Neurospora crassa

Abstract

Neurospora crassa is a eukaryotic organism belonging to the group of fungi called Ascomycetes. In nature, it grows on decaying or burnt vegetation in tropical or subtropical areas. Since Beadle and Tatum (1941) isolated the first biochemical mutants and suggested the “one gene-one enzyme hypothesis,” Neurospora has been a very popular model for biochemical genetics, molecular genetics, and for development (Russo and Pandit 1992).

Neurospora has several advantages for epigenetical studies: (1) it grows very fast; (2) it can grow on minimal defined medium, and (3) it produces many germinal cells such as conidia. A typical culture obtained from 20 ml of agar medium contains about 10⁹ conidia. Conidia contain on average two or three haploid nuclei. The mycelium is coenocytic, as the cells of the filaments forming the mycelium have dozens of nuclei. The organism is haploid except for a transient phase that precedes meiosis during the sexual cycle, and the haploid genome contains about 4 × 10⁷ base pairs on seven different chromosomes (Russo and Pandit 1992). Today, thousands of mutants of N. crassa are known and more than 620 genes have been mapped, and about 200 genes have been cloned (J. Kinsay [Director of Fungal Genetics Stock Center, Kansas City, Kansas], pers. comm.).

Neurospora is a heterothallic organism in that it has two distinct mating types, although these are morphogenetically indistinguishable. In the asexual life cycle there are three major morphological structures: the germinal cells called conidia; hyphae, which branch forming a mat called the mycelium; and...