GENESIS OF AN ORGAN - MOLECULAR ANALYSIS OF THE PHA-1 GENE

The organisation of organ formation is still an unsolved problem. Muta tions in the zygotic lethal gene pha-1 affect a late step during organ development in the nematode C. elegans. In mutant embryos all tissues in the pharynx fail to undergo terminal differentiation and morphogen esis. The expression of an early differentiation marker in pharyngeal muscle precursors is not impaired in mutant embryos, which suggests th at pharynx cells still acquire their identity. Therefore the gene defi nes an organ-specific terminal differentiation function. We cloned and sequenced the pha-1 gene and found that the deduced protein sequence contains features characteristic of the bZIP family of transcription f actors. During embryogenesis a transgenic pha-1 reporter construct is expressed transiently in all pharynx precursor cells at the time when these cells become restricted to form the pharynx organ. A mosaic anal ysis of the requirement of pha-1 activity during pharynx formation is consistent with the notion that pha-1 acts cell-autonomously in all ce lls of the pharynx primordium. The data suggest that pha-l initiates a nd coordinates programs required for cytodifferentiation and morphogen esis in all cell types of the entire organ on the transcriptional leve l. We propose that organs are independent developmental units whose id entity is reflected on the gene regulatory level.