In the free-living nematode Caenorhabditis elegans, the entire intestine is clonally derived from one cell of the 8-cell embryo. This makes gut an excellent lineage in which to study the early spatial control of gene expression in a "mosaic" embryo. We have used histochemistry to identify a nonspecific carboxylesterase activity that is a convenient biochemical marker for embryonic differentiation. The major gut esterase (the product of the gut esterase #1 or
ges-1 gene) has been purified to homogeneity and is a 60,000 dalton single polypeptide enzyme with a serine residue at the active site. The sequence of the cloned gene reveals significant homology with esterases from vertebrates and from insects. The genetic locus of the
ges-1 gene has been identified by means of induced isoelectric focusing mutants; these variants have in turn been used in a scheme to generate worms that completely lack
ges-1 activity. From an examination of these
ges-1 null embryos, we conclude that the
ges-1 esterase is essentially the only esterase that is expressed during the first half of embryogenesis.
ges-1 activity appears when the developing gut has 4-8 cells and the total embryo has only 100-150 cells. We show that the
ges-1 gene is transcribed from the zygotic genome, that
ges-1 expression is lineage autonomous, and that, after the embryo has divided once,
ges-1 expression is completely independent of cytokinesis. The DNA-polymerase inhibitor aphidicolin was used to show that
ges-1 expression depends neither on the nucleus:cytoplasm ratio of the embryo nor on the total number of rounds of DNA synthesis. Rather,
ges-1 expression responds in a manner that suggests a "quantal" cell cycle (i.e., a crucial period of DNA synthesis must occur during the cell cycle in which the gut is clonally established). This DNA synthesis somehow confers permission for a set of gut-specific genes to be transcribed up to several hours later. Molecular analysis of
ges-1 expression, in which mutated copies of the gene are transformed into the null mutants, is beginning to reveal some of the control circuitry of this simple developmental system. For example, deletions of the 5' flanking region of the
ges-1 gene can lead to esterase expression in both sister and cousin lineages, suggesting that spatial control of gene expression during development