Yeast genes were isolated that are required for restoring the osmotic gradient across the cell membrane in response to increased external osmolarity. Two of these genes, HOG1 and PBS2, encode members of the mitogen-activated protein kinase (MAP kinase) and MAP kinase kinase gene families, respectively. MAP kinases are activated by extracellular ligands such as growth factors and function as intermediate kinases in protein phosphorylation cascades. A rapid, PBS2-dependent tyrosine phosphorylation of HOG1 protein occurred in response to increases in extracellular osmolarity. These data define a signal transduction pathway that is activated by changes in the osmolarity of the extracellular environment.

Recently, a family of proteins containing the conserved motif Asp-Glu-Ala-Asp, the "DEAD box" proteins, has been identified. This family is typified by the eukaryotic translation initiation factor eIF4A, and its members are believed to share the functional property of ATP-dependent RNA unwinding. One of the previously identified members of this family (vasa) is the product of a maternally expressed gene from DrosophUa melanogaster that is known to play a role in the formation of the embryonic body plan. We report here the isolation of a Drosophila gene that has an mRNA expression pattern somewhat similar to that of vasa and also encodes a DEAD box protein. We have termed this gene ME3JB to reflect its maternal (ovarian germ-line) expression and its location within the 31B chromosome region. Comparisons with the other members of this family reveal that although ME31B is most like the protein Tifl/Tif2, which probably represents the Saccharomyces cerevisiae version of eIF4A, it is unlikely that ME31B represents the Drosophila eIF4A protein per se. A search for mutations in the ME3JB gene has established that the P element which causes the female-sterile mutation flipper lies in the 3' flank of the ME3JB gene.Recently a family of proteins that function as ATP-dependent RNA helicases has been identified (1).These proteins share a domain of about 370 amino acids in which a series of highly conserved motifs are arranged in identical order and with very similar spacing. Two of these conserved sequences represent specialized versions of the A and B motifs previously recognized in other ATP-binding proteins (2,3). The four-amino-acid sequence Asp-Glu-Ala-Asp (DEAD) is part ofthe specialized version ofthe B motif. These "DEAD box" proteins derive from organisms across all evolutionary orders (Escherichia coli to mouse), and for some of them, genetic and molecular studies have provided indications as to their functions in vivo (4-7). Thus this family of proteins appears to be involved in a wide range of intracellular events in which alteration of RNA secondary structure must play a critical role. These include formation of the initiation complex for mRNA translation, mRNA splicing, and ribosome biosynthesis. Functionally, the eukaryotic translation initiation factor eIF4A, which is the archetypal member of the family, is the one best understood at the molecular level (8-10).

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