Mut-5 is an endogenous mutator of Caenorhabditis elegans, causing Tc1 transposition and excision in the germline. Mut-5 was first mapped on chromosome 11 between
dpy-10 and
rol-1 (Mori et ai, 1988). Subsequent experiments showed that
mut-5 is located between
dpy-10 and
unc-4, to the left of
vab-9, on chromosome 11. In order to fine-map and identify
mut-5 a three factor cross between RP3 (
mut-5(
st701) unc- 4
(e120) 11:
unc-22(
st136::Tc1) IV) and RP5 (
dpy-10(
e128)
vab-9(
e1744) 11) was performed and a total of 12 Vab-9 Unc-22 recombinants was obtained. Mutator activity of the recombinants was tested by scoring for reversion of the
unc-22(
st136::Tc1) allele. 5 Vab-9 Unc-22 recombinants proved to be mutator positive and 7 mutator negative, which mapped
mut-5 near
zyg-11 on chromosome 11. Because the mutator is assumed to be a mobile element, the Vab-9 Unc22 recombinants were tested for polymorphisms cosegregating with
mut-5. Therefore chromosomal DNA of 9 Vab-9 Unc-22 recombinants (3 mutator positive and 6 mutator negative) was digested with EcoRI and analyzed in Southern blotting experiments. One of us (IM) identified 3 Tc1 elements cosegregating with
mut-5, Tc1#40, Tc1#55 and Tc1#118: these mapped in the area where
mut-5 was mapped by three factor crosses. Those elements were cloned and sequenced (IM) and their flanking sequences were used to investigate whether one of the Tc1 elements cosegregated with
mut-5 in the recombinants of the
dpy-10-
vab-9 area. Indeed one of them, Tc1#40, was present in all tested mutator positive recombinants and absent in the mutator negative recombinants. These results show that Tc1#40 maps in close proximity of
mut-5. We consider it likely that
mut-5 is Tc1#40. Since the sequence of this element shows no consistent differences with other Tc1 elements, Tc1#40 cannot be a mutator as a result of its sequence per se. Possibly the chromosomal position of Tc1#40 causes transcription of the element at the proper time and place for germline activation of Tc1. This hypothesis could also provide an explanation for the fact that Tc1 excises somatically in a non-mutator strain (Emmons et ai, 1983): copies of Tc1 could be transcribed in the soma but not in the germline. We are currently investigating this hypothesis by generating animals transgenic for Tc1#40 and flanking C. elegans sequences