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We have devised a simple method for identification of the 5'end of mRNAs in which the first strand cDNA is circularized and/or joined into a concatemeric form by T4 RNA ligase and the resulting single-stranded DNA is subsequently usedas atemplate for amplification of the 5'end by the polymerase chain reaction (1)(PCR) with gene-specific primers. The technique has been used for determination of the 5'end of Caenorhabditis elegans unc-13 and T12A2. 4 mRNAs.

Most clones isolated from cDNA libraries lack their 5'end, due to premature termination of cDNAsynthesis byreverse transcrip-tase. Therefore, 5'-end mapping ofmRNA is acrucial step for the analysis of a gene andits promoter. A variety of methods are available for mapping mRNA ends, including RNase protection, SI mapping and primer extension (2). These methods require relatively large amounts of mRNA and often pose difficulty in identifying the 5 …