Worm Breeder's Gazette 10(3): 72
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
We have been investigating the molecular basis of a mutator, which is required for germline transpositional activity of Tc1. The parallel of the genetic properties of the Tc1 mutator to autonomous elements in other eukaryotic element systems led us to look for a Tc1 co-segregating with the mutator. As we reported previously, we identified and cloned three 1.6 kb Tc1s (#40, #55, and #118) which cosegregate with mut-5 II, one of three mutators we identified. Three Tc1's map under mnDf88, and further, Tc1#40 and #55 map to a contig near lin-5 (determined by A. Coulson and J. Sulston), indicating the agreement of positions between cloned Tc1's and mut-5 locus. We further mapped mut-5 against Tc1#40, #55, and #118, by determining the presence or absence of these Tc1's as well as the mutator activity in Unc-4 recombinant lines derived from the mut-5 ( Tc1#55, #40, #118) rol-1/dpy-10 -22(st136::Tc1)/+ heterozygotes. Our data suggest that mut-5 is tightly linked to Tc1#55 and #40, but probably segregates out from Tc1#118. Because of the proximity of Tc1#55 and #40 (about 200 kb), we did not get recombination events between these two Tc1's to allow us to map further the position of mut-5. Phil Carter in Kemphues lab was interested in using Tc1#40 and #55 to map zyg-11 on a contig, and constructed Dpy-10 and Unc-4 recombinant lines from the similar cross described above. Fortunately, he got one Dpy-10 strain containing only Tc1#40 and one Unc-4 strain containing only Tc1#55. Phil kindly sent us his recombinant strains, and our analysis of the mutator activities of his strains suggests that the Unc-4 strain with Tc1#55 maintains mut-5 but the Dpy-10 strain with Tc1#40 does not. However, we have to mention here that the activity of the Unc-4 strain with Tc1#55 is somewhat lower than other mut-5 strains (about 10% of other mut-5 strains). When we constructed Unc-4 recombinant lines, we replaced the unc-22( st136::Tc1) region, which was always maintained in the mut-5 strains, by the N2 homolog. This replacement resulted in reducing the Tc1 copy number dramatically from 65 to 45. The examination of Tc1 patterns from these low copy number strains convincingly suggests that three Tc1's we cloned are the only Tc1's co-segregating with mut-5. Therefore, we propose that if the mutator is an active Tc1 element, it should be one of three, and most probably, is Tc1#55. We sequenced Tc1#55, #40, #118, and #150, an anonymous Tc1 from elsewhere, and compared them with two previously sequenced Tc1's, Ce( Be)Tl (Rosenzweig et al, 1983) and st137::Tc1 (Plasterk, 1987). The six Tc1's are different in 15 positions, but 5 of the differences come from st137::Tc1. When the polypeptide sequences of TcA, a putative transposase of Tc1, are compared (Table 1), Tc1#40, #55, and #150 have identical TcAs. The only specific change which distinguishes Tc1#55, the most probable candidate of the mutator, from other Tc1's is a G to T substitution at position 218, 5' of TcA. If Tc1#55 is the mutator, the sequence analysis provides two possible explanations for its mutator activity: one is a single nucleotide substitution, which would result in, for example, an elevated level of transcription of TcA in germline, and the other is a position effect. [See Figure 1]