Structural genes for catalytic and regulatory subunits of aspartate transcarbamylase

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.