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J. Biol. Chem., Vol. 280, Issue 21, 20268-20273, May 27, 2005

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Expression of Four Glutamine Synthetase Genes in the Early Stages of Development of Rainbow Trout (Oncorhynchus mykiss) in Relationship to Nitrogen Excretion^*

Phyllis A. Essex-Fraser, Shelby L. Steele, Nicholas J. Bernier, Brent W. Murray, E. Don Stevens, and Patricia A. Wright¶

From the Department of Zoology, University of Guelph, Guelph, Ontario N1G 2W1, Canada

The incorporation of ammonia into glutamine, catalyzed by glutamine synthetase, is thought to be important in the detoxification of ammonia in animals. During early fish development, ammonia is continuously formed as yolk proteins and amino acids are catabolized. We followed the changes in ammonia and urea-nitrogen content, ammonia and urea-nitrogen excretion, glutamine synthetase activity, and mRNA expression of four genes coding for glutamine synthetase (Onmy-GS01–GS04) over 3–80 days post fertilization and in adult liver and skeletal muscle of the rainbow trout (Oncorhynchus mykiss). Both ammonia and urea-nitrogen accumulate before hatching, although the rate of ammonia excretion is considerably higher relative to urea-nitrogen excretion. All four genes were expressed during early development, but only Onmy-GS01 and -GS02 were expressed at appreciable levels in adult liver, and expression was very low in muscle tissue. The high level of expression of Onmy-GS01 and -GS03 prior to hatching corresponded to a linear increase in glutamine synthetase activity. We propose that the induction of glutamine synthetase genes early in development and the subsequent formation of the active protein are preparatory for the increased capacity of the embryo to convert the toxic nitrogen end product, ammonia, into glutamine, which may then be utilized in the ornithine-urea cycle or other pathways.

Received for publication, November 1, 2004 , and in revised form, March 21, 2005.

^* This work was supported by a Natural Sciences and Engineering Research Council of Canada Discovery grant (to P. A. W.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Current address, Sciences Dept., Gaston College, Dallas, NC 28034.

Current address, Biology Program, College of Science and Management, University of Northern British Columbia, Prince George, British Columbia V2N 4Z9, Canada.

^¶ To whom correspondence should be addressed: Dept. of Zoology, University of Guelph, 50 Stone Road, Guelph, Ontario N1G 2W1, Canada. Tel.: 519-824-4120 (ext. 52719); Fax: 519-767-1656; E-mail: patwrigh{at}uoguelph.ca.

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