Isolation of cDNAs for trypsinogen from the winter flounder, Pleuronectes americanus.

Abstract

Knowledge of the timing of digestive enzyme expression in developing larvae is essential for evaluating the appropriateness of formulated larval diets. Since little is known at the molecular biological level about the ontogeny of digestive enzyme function in flatfish, we have isolated cDNA clones for key digestive enzymes such as trypsinogen. Portions of trypsinogen genes have been amplified from winter flounder cDNA libraries by PCR using primers based on sequence motifs conserved among trypsinogen genes from other organisms. The PCR products were sequenced, cloned, and used as radioactively labeled probes to screen the libraries. Three distinct trypsinogen cDNAs have been isolated, representing the first cDNAs for winter flounder digestive enzymes. The first type (Flounder 1) is very similar to a trypsinogen sequence reported from a related flounder, Pleuronectes platessa. The second type (Flounder 2) is related to sequences reported from Atlantic salmon, cod, and the Antarctic fish, Paranotothenia magellanica. The third type (Flounder 3) shows limited similarity to the Antarctic fish trypsinogen sequence. All three cDNAs encode a cleavable signal sequence at the amino-terminal end, and Flounder 1 and 2 contain the characteristic acidic sequence for activation of the trypsinogen proenzyme to trypsin. Southern hybridization experiments indicate that Flounder 1 and 3 are single-copy genes, whereas Flounder 2 may be present in more than one copy. In addition, Flounder 2 detects homologs in a wide variety of other fish species. The sequence information will be used to establish RT-PCR assays for larval winter flounder (Pleuronectes americanus) digestive enzyme expression.