Asako Sugimoto

Schizosaccharomyces pombe ste11 encodes a member of the family of HMG-box proteins. Its transcript is induced in response to nitrogen starvation and a concomitant decrease of the intracellular cAMP level. Expression of ste11 is essential for induction of sexual development, and its ectopic expression stimulates uncontrolled mating and sporulation. Ste11 protein regulates positively transcription of the following genes required for sexual development: the mating type genes, matP and matM, and the mei2 gene, which is essential for commitment to meiosis. Ste11 protein synthesized in vitro binds specifically to a DNA fragment carrying a 10-base motif TTCTTTGTTY that is an essential cis-acting element for the induction of mei2 and is commonly seen in the upstream regions of the genes inducible by nitrogen starvation. These observations strongly suggest that Ste11 serves as a key transcription factor for sexual development.

The endoderm in the nematode Caenorhabditis elegans is clonally derived from the E founder cell. We identified a single genomic region (the endoderm-determining region, or EDR) that is required for the production of the entire C. elegans endoderm. In embryos lacking the EDR, the E cell gives rise to ectoderm and mesoderm instead of endoderm and appears to adopt the fate of its cousin, the C founder cell. end-1, a gene from the EDR, restores endoderm production in EDR deficiency homozygotes. end-1 transcripts are first detectable specifically in the E cell, consistent with a direct role for end-1 in endoderm development. The END-1 protein is an apparent zinc finger-containing GATA transcription factor. As GATA factors have been implicated in endoderm development in other animals, our findings suggest that endoderm may be specified by molecularly conserved mechanisms in triploblastic animals. We propose that end-1, the first zygotic gene known to be involved in the specification of germ layer and founder cell identity in C. elegans, may link maternal genes that regulate the establishment of the endoderm to downstream genes responsible for endoderm differentiation.

We cloned the myo2 gene of Schizosaccharomyces pombe, which encodes a type II myosin heavy chain, by virtue of its ability to promote diploidization in fission yeast cells. The myo2 gene encodes 1,526 amino acids in a single open reading frame. Myo2p shows homology to the head domains and the coiledcoil tail of the conventional type II myosin heavy chain and carries putative binding sites for ATP and actin. It also carries the IQ motif, which is a presumed binding site for the myosin light chain. However, Myo2p apparently carries only one IQ motif, while its counterparts in other species have two. There are nine proline residues, which should break alpha-helix, in the COOH-terminal coiled-coil region of Myo2p. Thus, Myo2p is rather unusual as a type II myosin heavy chain. Disruption of myo2 inhibited cell proliferation. myo2Delta cells showed normal punctate distribution of interphase actin, but they produced irregular actin rings and septa and were impaired in cell separation. Overproduction of Myo2p was also lethal, apparently blocking actin relocation. Nuclear division proceeded without actin ring formation and cytokinesis in cells overexpressing Myo2p, giving rise to multinucleated cells with dumbbell morphology. Analysis using tagged Myo2p revealed that Myo2p colocalizes with actin in the contractile ring, suggesting that Myo2p is a component of the ring and responsible for its contraction. Furthermore, genetic evidence suggested that the acto-myosin system may interact with the Ras pathway, which regulates mating and the maintenance of cell morphology in S. pombe.