The GFP reporter was strongly expressed in the pharynx and only the pharynx. Cells in both the anterior and posterior pharynx express the reporter gene. Rectal staining was never observed. Analysis of confocal sections identify the 20-30 expressing cells as primarily muscle and marginal cells; staining in other classes of pharynx cells(e.g., epithelial, gland or nerve cells, as well as cells of the pharyngeal-intestinal valve) was much weaker and more variable. Interestingly, expression in muscle cells of the
m6class was much weaker than in the neighboring classes
m5and
m7; this expression pattern is reminiscent of that seen with the homeobox gene
ceh-22 (Okkema and Fire, 1994) and indeed, several potential CEH-22 binding sites can be identified in the enhancer sequence (P.Okkema, personal communication).