THE CAENORHABDITIS-ELEGANS NK-2 CLASS HOMEOPROTEIN CEH-22 IS INVOLVEDIN COMBINATORIAL ACTIVATION OF GENE-EXPRESSION IN PHARYNGEAL MUSCLE

The pharyngeal muscles of Caenorhabditis elegans are single sarcomere muscles used for feeding. Like vertebrate cardiac and smooth muscles, C. elegans pharyngeal muscle does not express any of the known members of the MyoD family of myogenic factors. To identify mechanisms regula ting gene expression in this tissue, we have characterized a pharyngea l muscle-specific enhancer from myo-2, a myosin heavy chain gene expre ssed exclusively in pharyngeal muscle. Assaying enhancer function in t ransgenic animals, we identified three subelements, designated A, B an d C, that contribute to myo-2 enhancer activity. These subelements are individually inactive; however, any combination of two or more subele ments forms a functional enhancer. The B and C subelements have distin ct cell type specificities. A duplication of B activates transcription in a subset of pharyngeal muscles (m7, m3, m5 and m7). A duplication of C activates transcription in all pharyngeal cells, muscle and non-m uscle. Thus, the activity of the myo-2 enhancer is regulated by a comb ination of pharyngeal muscle-type-specific and organ-specific signals. Screening a cDNA expression library, we identified a gene encoding an NK-2 class homeodomain protein, CEH-22, that specifically binds a sit e necessary for activity of the B subelement. CEH-22 protein is first expressed prior to myogenic differentiation and is present in the same subset of pharyngeal muscles in which B is active. Expression continu es throughout embryonic and larval development. This expression patter n suggests CEH-22 plays a key role in pharyngeal muscle-specific activ ity of the myo-2 enhancer.