A PCR Procedure to Determine the Sequence of Large Polypeptides by Rapid Walking Through a cDNA Library

A procedure that uses the PCR to make rapid successive steps through a random-primed cDNA library has been developed to provide a method for sequencing very long genes that are difficult to obtain as a single clone. In each successive step, the portions of partial clones that extend out from the region of known DNA sequence are amplified by two stages of PCR with nested, outward-directed primers designed ≈50 bases in from the end of the known sequence, together with a general primer based on the sequence of the vector. This procedure has been used to determine the coding sequence of the cDNA for the β heavy chain of axonemal dynein from embryos of the sea urchin Tripneustes gratilla. By starting from a single parent clone, whose translated amino acid sequence overlapped the microsequence of a tryptic peptide of the β heavy chain, and making 3 such walk steps downstream and 14 walk steps upstream, we obtained a sequence of 13,799 base pairs that had an open reading frame of 13,398 base pairs. This sequence encodes a polypeptide with 4466 residues of Mr511,804 that is believed to correspond to the complete β heavy chain of ciliary outer arm dynein.