Transfer of the mutation’ of into 168-type strains does not always lead to simultaneous induction of both chromosomal aberrations (translocation and inversion) of the strains carrying it. A strain was isolated (GSY1835) which possessed new rearrangements. It has some characteristics of the donor parent-a mutation in the locus () which does not revert, a similar effect on tryptophan biosynthesis and loss of linkage between the markers flanking the operon. Unlike the ‘trpE26 strains’, GSY1835 yielded haploid Trp transformants and transductants. Merodiploid Trp clones were also produced, but in a fairly low proportion (about 10%). The genetic structure of GSY1835 was found to differ considerably from that of a ‘ strain’. The strain carries a non-tandem duplication of part of segment I of the chromosome (segment Ib: ), with the two copies in inverted position. This duplication is very stable. Recombination may occur betweeen the two copies. The results fit two configurations for the genetic map of the strain and its derivatives. Map 1 differs from that of strain 168 by the ‘insertion’ of the second copy of Ib inside the locus. In map 2, introduction of this second copy at the same site is accompanied by an inversion of the lower part of the chromosome (segments C-D-II). The haploid Trp clones have the genetic map of strain 168. Models are proposed to interpret the formation of strain GSY1835 () and the production of the haploid Trp transformants and transductants.


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