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This Article Full Text (PDF) All Versions of this Article: jas.2007-0159v1 86/9/2288    most recent Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Connors, M. T. Articles by Cant, J. P. Search for Related Content PubMed PubMed Citation Articles by Connors, M. T. Articles by Cant, J. P.

Protein elongation rates in tissues of growing and adult sheep

M. T. Connors^*, D. P. Poppi^* and J. P. Cant

^* Schools of Animal Studies and Veterinary Science, University of Queensland, St Lucia 4072, Brisbane, Australia Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario, N1G 2W1, Canada

d.poppi{at}uq.edu.au

Abstract

To identify the relative roles of translation initiation and elongation in the long-term control of protein synthesis in ovine tissues, fractional synthesis rates (FSR) and ribosomal transit times (RTT) were measured in vivo in 24 ewe lambs at 3 levels of intake (maintenance (M), 1.5M, and 2M) and 8 mature ewes at 2M intake. After 17 to 25 d on treatment, animals were given an i.v. flooding dose of L-[ring-2,6-³H]phenylalanine and tissues were collected for analysis of radioactivity in free, total protein, and nascent ribosome-associated proteins. Ribosome transit time, which is the inverse of elongation rate, averaged 83, 393, 183, 241, 85 and 113 s for liver, duodenum, skin, rumen, semimembranosus, and longissimus, respectively. In response to increased level of intake, protein FSR increased (P < 0.01) in all tissues but rumen and was attributed to greater translational efficiency. There was no effect (P > 0.50) of intake on RTT in these tissues and the estimated proportion of ribosomes attached to and actively translating mRNA was increased (P < 0.07), indicating that an up-regulation of initiation was responsible for the faster FSR. Mature ewes exhibited lower (P < 0.10) protein FSR in all tissues compared to lambs, which was related to a decline in the RNA:protein ratio in all tissues except for liver and duodenum. In all tissues but liver and semimembranosus, RTT increased (P < 0.10) with age. The lower elongation rate was not considered to have influenced the protein synthetic rate, but it caused an increase in the proportion of ribosomes actively translating mRNA. It is anticipated that this work will provide direction to future studies of the molecular mechanisms of chronic FSR control.

Key Words: ribosome transit time • protein synthesis • sheep • intake • age