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Figure 9 Development regulation of splicing of xSlo transcripts. The xSlo gene of X. laevis displays both tissue-specific and developmentally regulated splice variants. a, Four alternative splice variants - xSlo15, xSlo56, xSlo59, and xSlo99 - were detected by reverse transcriptase polymerase chain reaction (RT-PCR) analysis of RNA isolated from 2-day-old Xenopus embryos. Three of the alternatively spliced variants were expressed in both embryonic spinal cord and somites. One variant, xSLO59, was only detected in spinal cord. Constitutively expressed EF-1a and N-tubulin served as positive controls. Examination of 1- or 2-day-old embryo RNA indicated that the xSLO59 variant was developmentally upregulated (b) in vivo and (c) in vitro. Reproduced from Kukuljan, M., Taylor, A., Chouinard, H., Olguin, P., Rojas, C. V., and Ribera, A. B. 2003. Selective regulation of xSlo splice variants during Xenopus embryogenesis. J. Neurophysiol. 90, 3352-3360, used with permission from The American Physiological Society.

predict truncated proteins. Whether or not the predicted truncated proteins are expressed and what their potential roles are remain unknown. However, the existence of these splice variants that are devel-opmentally regulated raises the interesting possibility of negative-feedback mechanisms implemented by alternative splicing. Developmentally regulated variants of calcium channels that code for truncated ion channel proteins have also been observed (Okagaki et al., 2001; Figure 10).

Putative splicing point

TuCa1 TCGTTAGCCA CCGAACATTA CAGACAACCG CCATGGCTGA CATTAGCGCA AGACTTGGCG AACAAGATCC TCTTAACACT CTTCACTATC GAGATGCTCG SLATEHY RQPPWLTLAQDLANKILLTLFTIEML

II S1 Nonsense codon | ii S2 1st Met

ANTuCa1 GTTACAAGGA TATTCTGCCA ATATAGGGTG TCTCTATAAC GTCTCAATTG TGACTTGGCG AACAAGATCC TCTTAACACT CTTCACTATC GAGATGCTCG

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