Understanding how basic helix-loop-helix (bHLH) transcription factors cause changes in gene expression is important for revealing how transcriptional cascades function in development. Several bHLH transcription factors such as MyoD and NeuroD have been shown to play important roles

Understanding how basic helix-loop-helix (bHLH) transcription factors cause changes in gene expression is important for revealing how transcriptional cascades function in development. Several bHLH transcription factors such as MyoD and NeuroD have been shown to play important roles in promoting myogenesis and neurogenesis, respectively (for a review, see Massari and Murre, 2000). These transcription factors work as homoor heterodimers through contacts made in the helix-loop-helix (HLH) domain. The dimers then bind promoter DNA at canonical sequences called E boxes by virtue of the now close proximity of their individual basic DNAbinding domains (Murre et al., 1989). In general, bHLH heterodimers are composed of one member with tissue-specific expression and another member with more broad expression throughout the organism (Lassar et al., 1991). One tissuespecific bHLH family member, Twist, plays a role in the development of the mesoderm in all organisms examined including humans, mice, C. elegans and Drosophila melanogaster (where the gene was first discovered) (Simpson, 1983; Nüsslein-Volhard, 1984; Chen and Behringer, 1995; Wang et al., 1997; Harfe et al., 1998b; Corsi et al., 2000). In several organisms, the broadly expressed bHLH family members, E (in vertebrates) and Daughterless (in invertebrates), are thought to heterodimerize with Twist (Spicer et al., 1996; El Ghouzzi et al., 2000; Castanon et al., 2001). In C. elegans, the E/Daughterless (DA) homolog is called CeE/DA and, although embryonic expression is ubiquitous, the protein has a restricted expression pattern postembryonically that includes mesodermal tissue expressing CeTwist (Krause et al., 1997). We are interested in elucidating aspects of mesodermal development and have been studying the function of Twist in the model organism C. elegans. Of the 35 conventional bHLHs identified in the C. elegans genome, a single Twist homolog exists (Ruvkun and Hobert, 1998; Ledent and Vervoort, 2001). Initially, we characterized a putative null allele of the CeTwist gene, hlh-8 and found several defects in the mesoderm of the mutant animals (Corsi et al., 2000). The C. elegans mesoderm is predominantly composed of body wall muscles used for movement, macrophage-like 2761 Development 129, 2761-2772 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 DEV14519