Worm Breeder's Gazette 16(4): 32 (October 1, 2000)
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.
Department of Biochemistry and Molecular Biology, Louisiana State University Health Sciences Center, Shreveport, LA 71130-3932
We have identified a 104-bp sequence in the mec-3 5’ control region, that when present in high copy extrachromosomal arrays, causes ALM cell migration defects and PLM axonal defects. The ALM migration defects were not due to RNAi, nonspecific effects of carrying a transgenic array, expression of GFP, or the rol-6 marker gene used to make the transformants. mec-3 is a master regulator of the touch receptor cell fate and both the ALMs and the PLMs are touch receptor neurons. This sequence appears to be sequestering a specific DNA binding factor or factors. In some cases, titration of this factor reversed the direction of ALM migration, so this factor may be involved in controlling both the direction and extent of ALM migrations.
The PLM axonal defects correlate well with the ALM migration defects (i.e. arrays that cause a high percent of the ALMs to migrate aberrantly also cause a high percent of the PLMs to have axonal defects), so the PLM defects are probably induced by titration of the same factor or factors that control ALM migrations. In both cases, the cells may fail to correctly interpret anterior-posterior spatial information. However, the ALMs migrate to more anterior positions, whereas the PLM axons often terminate at more posterior positions.
The factor sequestered by the titrating sequence does not affect all cell migrations. The AVM touch receptor neurons are correctly positioned in strains that show strong ALM migration defects. The AVM's lineal mother and grandmother both migrate anteriorly and, if either of these migrations were defective, the AVMs would be mispositioned.
As in sea urchin embryos, titration of transcription factors with high copy transgenes may be useful for characterizing control elements in C. elegans. We do not know, at this point, how often transcription factors will be present at low enough concentrations and bind DNA with enough affinity to apply this approach. Others have reported competitive titration by transgenic sequences in C. elegans (1, 2), so we expect that it will not be extremely uncommon. Certainly some of the artifacts associated with transformation of C. elegans with high copy arrays may result from titration of DNA binding factors. Transformation of C. elegans with low copy arrays made with genomic carrier DNA a la Andy Fire (3) did not cause migration defects, so the use of low copy arrays may be prudent where titration effects are not desired.
1. I. Carmi, J. B. Kopczynski, B. J. Meyer, Nature 396, 168-173 (1998).
2. W. Li, A. Streit, B. Robertson, W. B. Wood, Genetics 152, 237-248 (1999).
3. W. G. Kelly, S. Xu, M. K. Montgomery, A. Fire, Genetics 146, 227-238 (1997).