Short communication Molecular analysis of a s1 -, b-, a s2 - and k-casein transcripts reveals differential translational efficiency in yak lactating mammary gland W.L. Bai a,d , R.H. Yin a,n , W.Q. Jiang a , O.O. Ajayi d , S.J. Zhao b , G.B. Luo a , Z.H. Zhao c,n , I.G. Imumorin d a College of Animal Science and Veterinary Medicine, Shenyang Agricultural University, Shenyang 110866, China b Institute of Biotechnology, Animal Science Research Academy of Sichuan Province, Chengdu 610066, China c College of Animal Science and Veterinary Medicine, Jilin University, Changchun 130062, China d Department of Animal Science, Cornell University, Ithaca, NY 14853, USA article info Article history: Received 29 August 2012 Received in revised form 6 December 2012 Accepted 7 December 2012 Keywords: Yak Casein Transcriptional abundance Translational efficiency abstract Caseins are important for cheese making, and their amounts and relative concentrations have important influence on the nutritional and technological properties of milk. In the present work, we investigated the mRNA relative proportions and translational efficiency of yak casein transcripts and showed that, for the first time, the four casein mRNAs were not transcribed and translated with the same efficiency. The mRNA transcripts for yak a s1 -casein (17.5%) and k-casein (20.9%) appeared to be less abundant than those of b-casein (31.9%) and a s2 -casein (29.7%). The quantitative determination of the four casein fractions of yak milk showed that b-casein and a s1 -casein were the main caseins, accounting for 48.2% (16.5 g/L) and 30.8 (10.5 g/L), respectively, whereas the a s2 -casein (8.7%, 2.9 g/L) and k-casein (12.3%, 4.2 g/L) exhibited less abundance. The a s1 -casein transcripts had the highest translational efficiency with a value of 1.8, and the next highest was b-casein transcripts of which the value was 1.5, whereas the a s2 -casein transcripts had the lowest translational efficiency with a value of 0.3. The analysis results of the sequence context of translation initiation codon AUG of the casein mRNA might explain, at least in part, the differential transcriptional and translational rate observed among the casein transcripts. The results from the present work would contribute to elucidating the molecular regulatory mechanisms of yak casein translation. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Caseins are important for cheese making, and their amounts and relative concentrations have important influence on the nutritional and technological proper- ties of milk (Wedholm et al. 2006). In ruminant, caseins comprise a family of four peptide chains including a s1 - casein, b-casein, a s2 -casein and k-casein (Bevilacqua et al., 2006). The four caseins are coded by four tightly linked genes in order CSN1S1, CSN2, CSN1S2 and CSN3 (Rijnkels et al., 1997), and the order and transcriptional orientation of the genes are highly conserved among ruminant species (Dovc, 2000). In yak milk, the caseins comprise approximately 84% of the total protein content, being higher than its cow counterpart (Wang and Zou, 1995; Huppertz et al., 2006). Regulation of gene expression is achieved through a series of complex mechanisms at different levels, includ- ing transcription, pre-mRNA splicing, mRNA stability, and translation, etc. In several ruminant species, studies demonstrated that the expression of caseins was also Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/livsci Livestock Science 1871-1413/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.livsci.2012.12.008 n Corresponding author. E-mail addresses: yinronghuan@163.com (R.H. Yin), zh65zh@yahoo.cn (Z.H. Zhao). Livestock Science 152 (2013) 74–78