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A cDNA library of newborn rat brain poly(A)+ RNA in phage lambda gt11 was screened with a polyclonal antibody against chicken DNA polymerase beta. One positive phage was isolated and purified after testing 2 X 10(7) recombinants. This phage, designated lambda pol beta-10, contained an 1197-base-pair cDNA insert that corresponded to a mRNA with a poly(A) sequence at the 3' terminus and a single, long open-reading frame of 957 bases. The open-reading frame, starting 44 residues from the 5' end of the cDNA, predicted a 36,375-Da protein of 318 amino acids. Comparison of this deduced amino acid sequence with the partial sequence obtained with purified polymerase beta revealed a match of six tryptic peptides, involving a total of 47 amino acid residues. This confirmed the identity of the cDNA. Blot-hybridization analysis of newborn rat brain poly(A)+ RNA revealed a mRNA species of approximately …