A screen of gene expression patterns has been developed for the nematode Caenorhabditis elegans. Promoter-reporter gene fusions were constructed in vitro by ligating C. elegans genomic DNA fragments upstream of a lacZ gene. Patterns of beta-galactosidase expression were examined by histochemical staining of C. elegans lines transformed with the constructs. beta-galactosidase expression depended on translational fusion, so constructs were assayed in large pools to expedite detection of the low proportion that were active. Expression in a variety of cell types and temporal patterns was observed with different construct pools. The most striking expression patterns were obtained when the beta-galactosidase activity was localized to subcellular structures by the C. elegans portion of the fusion protein. The active constructs of three selected pools were identified subsequently by an efficient combinatorial procedure. The genomic locations of the DNA fragments from the active constructs were determined and appear to define previously uncharacterized genetic loci.

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