Cloning and expression of the quaking gene in the zebrafish embryo.

The responsible gene for hypomyelinating quaking deficiency, qkI, encoding a KH RNA binding protein, is expressed abundantly in the developing mouse nervous system, whereas who/how/struthio, a homologue of the qkI in Drosophila, is expressed predominantly in the mesoderm. Here we describe the isolation and early developmental expression of a zebrafish homologue of qkI. The zebrafish quaking cDNA, zqk, exhibits striking conservation with qkI across the coding region, accompanied by a unique 123 nucleotide insertion sequence. Maternal and zygotic zqk transcripts are ubiquitously distributed during cleavage and blastula periods, and then accumulate in the dorsal midline of the body trunk during gastrulation. During segmentation and pharyngula periods zqk transcripts are expressed in the neural tissue of the head region, and in the paraxial mesoderm of the body trunk. Subsequently they diminish until the hatching period, when they are expressed only in the cardiac sac and pectoral finbuds. We also found that the zqk transcript is alternatively spliced with the transcript containing a 123 nucleotide additional segment localized in neural tissue in the head region, but not in the paraxial mesoderm in the body trunk. The data suggest that the quaking gene family originated in the mesoderm and evolved to become expressed in the nervous system in lower vertebrates. The insertion of the 123 nucleotide sequence could be related to the acquisition of a neural function for the gene.