EXPRESSION OF CARBAMOYL-PHOSPHATE SYNTHETASE-III MESSENGER-RNA DURINGTHE EARLY STAGES OF DEVELOPMENT AND IN MUSCLE OF ADULT RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

It has been reported that the activities of the urea cycle related enz ymes ornithine carbamoyltransferase and carbamoyl-phosphate synthetase III (CPSase III) are induced during early life stages of ammonotelic rainbow trout (Oncorhynchus mykiss), suggesting that the urea cycle ma y play a physiological role in early development in teleost fish (Wrig ht, P. A., Felskie, A., and Anderson, P. M. (1995) J. Exp. Biol. 198, 127-135). CPSase III cDNA prepared from embryo mRNA was sequenced, con firming the existence of the CPSase III gene in trout and its expressi on. The deduced amino acid sequence of the CPSase III is homologous to other CPSases. Supporting evidence for the expression of CPSase III a ctivity in trout embryos was obtained by demonstrating expression of C PSase III mRNA as early as day 3 post-fertilization, reaching a maximu m at 10-14 days, declining to a minimum at day 70, and then increasing to a relatively constant level from days 90 to 110 (relative to total RNA). Unexpectedly, in tissues of adult and fingerling trout, CPSase III mRNA was found to be present in muscle but not in other tissues, i ncluding liver. This finding was confirmed by assay of extracts, which showed CPSase III and ornithine carbamoyltransferase activity in musc le but not in other tissues. The pyrimidine nucleotide pathway-related CPSase II mRNA was expressed in all tissues.