Over forty aspartate transcarbamylase mutants (pyr B) of Salmonella typhimurium have been surveyed. The mutants have been classified as to mutational type and used to construct a genetic map of the pyr B locus. The direction of transcription across this locus is inferred from the fact that several amber nonsense mutations at one extreme end of the map leave the cell with residual pyr B function. Seven nonsense and frameshift mutants of the pyr B gene were selected and assayed for their content of regulatory subunit; all possessed reduced but detectable levels of the subunit. Since pyr B mutants lack catalytic activity but possess detectable regulatory subunit we conclude that the pyr B locus codes only for the catalytic subunit. Two deletion mutants were isolated that lack the entire pyr B locus and produce no detectable regulatory subunit; we infer that the structural gene for regulatory subunit is adjacent to the pyr B locus. The reduced level of regulatory subunit in pyr B mutants is consistent with a polarity effect of pyr B mutations on an adjacent gene for this subunit. No point mutations of the regulatory subunit structural gene have been found.
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