myo-2 encodes a myosin heavy chain isoform expressed specifically in pharyngeal muscles. We have been analyzing
myo-2 regulatory regions using lacZ fusions. Our experiments have been done by examining -gal expression in L3-adult F1 progeny of injected hermaphrodites. We have identified an enhancer in a 227bp EcoRI-RsaI fragment located about 670bp upstream of the
myo-2 translational start site ( ATG). When inserted upstream of a
glp-1/lacZ fusion, this enhancer induces -gal activity almost exclusively in the pharyngeal muscles (a small number of animals (<4%) also express -gal in 1-3 large nuclei near the anus; we have not characterized these cells further). The
glp-1/lacZ fusion alone is not expressed. Therefore, the
myo-2 enhancer appears almost completely specific for pharyngeal muscles. Consistent with this result, both
myo-3/lacZ and CeMyoD/lacZ fusions, which are normally active only in body wall muscle, are expressed in pharyngeal and body wall muscles when placed downstream of the
myo-2 enhancer.
myo-2/lacZ fusions lacking the enhancer are also expressed in pharyngeal muscle. Thus, there are at least two independent pharyngeal specific regulatory elements upstream of
myo-2. This downstream element has been localized to 250bp NarI-PvuI fragment just 5' to the ATG translational initiation codon. It does not act as an independent pharyngeal enhancer when inserted upstream of the
myo-3 promoter. Rather, this element behaves as a pharyngeal muscle specific promoter.
myo-2 regulation is analogous to that of
unc-54 (Fire and Harrison, WBG11,2,p.22). In that case, a body wall specific element just upstream of the promoter acts only very weakly as an enhancer, while an internal element 1kb downstream acts independently as a strong body wall muscle specific enhancer. We are currently defining the pharyngeal muscle regulatory sequences more precisely and plan to identify trans-acting factors interacting with them. [See Figure 1]