We have obtained a new mutator (activator of transposition) in a line of primarily Bristol N2 background, using EMS as a mutagen. This new mutator locus, called
mut-7 , is located on chromosome III and activates Tc1, Tc3, Tc4 and Tc5 in the germ line. Somatic transposition activity does not seem to be affected. The method we used to detect mutators was in principle the same as the one used by John Collins (1). However, instead of searching for an elevated transposition frequency, we set out to find mutators in a background that had no transposition activity at all. The
unc-54(
r323::Tc1) allele used in this screen is derived from TR674 (a high hopper strain) that was backcrossed 10 times with Bristol N2, after which the
unc-54(
r323::Tc1) allele did not revert anymore. Judged from a southern blot this strain only has about 4 extra Tc1 copies compared to the 30 of Bristol N2. Worms were mutagenised, cloned out, and checked for the occurrence of non-Unc progeny. Among 1000 mutagenised animals one line was found in which Tc1 could excise. The excision frequency from the
unc-54 locus is similar to that of TR674, a
mut-2 strain. Not only Tc1 excision but also integration frequencies are raised: spontaneous, revertible
unc-22 mutations can be identified. Also other transposons (Tc3, 4 and 5) proved to be activated, as scored by reversion of the respective transposon insertion alleles of
unc-22 (kindly provided by John Collins). Southern blot experiments show that upon culturing, new alleles of Tc1 and Tc3 arise. Activation of transposition seems to be specific for the germ-line, since no elevation of somatic transposition activity could be detected using a PCR assay. Crosses with Bristol N2 show the mutation to be a single locus in the centre of chromosome III; it will be named
mut-7 . Further mapping studies are now in progress to clone the
mut-7 locus. It may be interesting to note that the
mut-7 allele also confers a Him-phenotype, just like
mut-2 and the other EMS induced mutators found by John Collins. The fact that this new high hopper line has a Bristol background makes it a much healthier strain than the
mut-2 strains. For example, lines with
mut-7 can readily be injected to generate transgenic lines, unlike in our hands other high hopper strains. This, combined with the fact that there are only a few extra transposons compared with Bristol N2, makes that these lines are very useful for transposon tagging experiments. Together with the ongoing sequencing project, this should make it possible to quickly identify new mutations. 1. J. Collins, B. Saari and P. Anderson (1987). Nature <b>238 726-728