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The close linkage of the glnA gene with polA was exploited to construct a fine structure map of polA by means of generalized transduction with phage P1. Nine different polA- alleles were mapped by recombinational crosses. The results indicate a gene order consistent with previous observations (KELLEYand GRINDLEY 19 76a; MURRAY an d KELLEY 1979). Three mutations, polA5, polA6 and polA12 map within the "carboxy-terminal" or "large-fragment'' portion of the gene in unambiguous order. Four alleles, known to affect the "aminoterminal" portion of the gene, polA107, polA214, polA480ex and polA4113, appear to be closely linked with certain ambiguities in their exact order. All four of these mutations are known to alter the 5'→3' exonuclease activity of DNA polymerase I and three of them result in the conditional lethal polA- phenotype. The polA1 nonsense mutation maps between these two groups in a position consistent with its known effect, production of an amber fragment that includes the 5'→3' exonuclease. The final allele, resAl, is another nonsense mutation that maps at; the extreme "amino-terminus" of the cistron.—A number of control experiments were conducted to determine the effects of pol.A- mutatims on the PI-mediated recombinational event. These experiments indicated that abortive transduction occurs quite frequently, but the formation of abortive transductants and segregation of unselected transduced markers among daughter progeny is like that observed by other investigators. There was no evidence that any individual polA- allele behaved in an exceptional fashion during recombination.

MAPPING OF THE polA LOCUS OF ESCHERICHIA COLI K12: GENETIC FINE STRUCTURE OF THE CISTRON