"Position effect" on dominance in the D-serine deaminase system of Escherichia coli K-12.

A completely stable F32 merodiploid has been isolated, which has the dsd genotype dsdA(+)dsdCx3 /F -dsdA(+)dsdC(+). The stabilization apparently does not result from an alteration in the F, or from its integration in the chromosome, but rather from a chromosomal aberration. The stable strain can exist in two forms, which interconvert at low frequency. In one form, the episomal dsd markers are in normal apposition with the chromosomal markers, and in this case cis dominance of dsdCx3 is incomplete. In the other form, the two regions are not in normal apposition, and dsdCx3 is completely cis dominant. A model to explain these findings is presented.