The Caenorhabditis elegans single tropomyosin gene,
tmy-1/lev-11 generates four isoforms CeTMI, II, III and IV- by combining differential promoter selection and alternative splicing. CeTMI and CeTMII are expressed in body wall, anal and sex muscles from the external promoter whereas CeTMIII and CeTMIV are expressed in pharynx and intestines from the internal promoter. CeTMIII is further expressed in the germline. To identify factors involved in expression regulation of isoforms CeTMIII and CeTMIV, the internal promoter of
tmy-1 was analyzed. Promoter deletion assay showed that the regulatory element for CeTMIV expression was 300 to 505 bp upstream of the transcription start codon. A factor, yet to be identified near the PHA-4 site of the internal promoter, may cooperate to control expression of CeTMIV in pharynx and intestine. Within the 505bp minimum promoter required for expression of CeTMIV are two GATA binding sites in the reverse direction and one GATA-like site. The GATA-like site contributed to intestinal expression but not pharyngeal expression as shown by mutation of the GATA-like site. This GATA-like site alone without an upstream region including the PHA-4 binding site was not sufficient for expression of CeTMIV in both pharynx and intestine. In a yeast one-hybrid screen, EGL-18 bound to a bait taken from the internal promoter. Knock out of EGL-18 by RNA-mediated interference resulted in embryonic lethality and L1 larva that survived died at the L1 stage. CeTMIV reporter genes were not expressed in the L1 larva that survived the
egl-18 RNA-mediated interference treatment. Consistent with RNAi knockout,
egl-18(
mt162) and
egl-18(
ew34) mutant worms did not express CeTMIV reporters. Although we did not confirm binding EGL-18 with any of the GATA sites in the internal promoter of
tmy-1, CeTMIV expression could require the presence of EGL-18. These results suggest that EGL-18 controls transcription of
tmy-1 from the internal promoter.