ABSTRACT The saliva of various animals contains praline-rich proteins which may play important roles in prevention of mineral precipitation, protection of dietary and digestive proteins from interaction with tannins, and modulation of bacterial colonization on the tooth surface. Previously, we found a segment of Escherichia coli genomic DNA in bovine tooth germ mRNA encoding the proline-rich protein P-B. To examine whether E. coli genomic DNA is present in bovine genomic DNA, we constructed a plasmid library from the bovine DNA. Although results so far have failed to indicate any such presence in the bovine nucleotides examined, experiments using the polymerase chain reaction (PCR) revealed unusual amplification of nucleotides. As an initial step of the study on possible occurrence of E. coli-derived nucleotide sequence in bovine genomic DNA of P-B, we examined the structure of the PCR products generated by unexpected amplification. The determined structure of the PCR products suggested that when the two single strand chains that grow by reading the sequence of the respective template reached a hybridizable short nucleotide structure, they became hybridized and subsequent elongation was continued by reading the sequence of the counter chain that had been elongated by reading the template. It is possible that elongation of the chain was interrupted once before the completion of amplification due to the template’s palindrome region which had formed a double strand structure during the PCR process. Such an unusual amplification made possible under certain conditions in a DNA sequence may be one of the mechanisms for the genetic recombination found in our previous study.
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