Mutations in the gene
mec-7 result in touch-insensitive animals whose touch cell processes lack the 15-protofilament microtubules normally present in and specific to the touch cells. The
mec-7 gene has been cloned and sequenced, and appears to encode a -tubulin which is necessary for the formation of these microtubules. One of the cloned C. elegans -tubulin genes (originally called tub- 2) is contained in a contig which maps by in situ hybridization to the region of the X chromosome near
mec-7. We have used the cosmid R02A8 containing this -tubulin gene and a subfragment of this cosmid to probe genomic DNA from wild type and from
mec-7 mutants. DNAs from seven
mec-7 mutants exhibit restriction fragment length differences when probed with R02A8. Five mutants, two from EMS mutagenesis (
e1505 and
u156) and three from gamma-ray mutagenesis (
u443,
u448, and
u453), contain deletions. The EMS mutations delete approximately 600bp and 300bp, respectively, of DNA from a fragment containing -tubulin homology. Two of the gamma-ray mutations (
u443 and
u448) delete approximately 28kb and 18kb, respectively, of DNA that includes the region of -tubulin homology. Animals homozygous for these mutations are completely touch insensitive but show no other behavioral defects. The third gamma-ray-induced mutation (
u453) deletes more than 40kb of DNA and results in additional defects (partial lethality at 25 C and poor male mating). These additional phenotypes are probably associated with defects in one or more adjacent genes. DNAs from two other mutants (
u382 and
u388, both derived from TR679) contain insertions of approximately 3.5kb into the -tubulin sequences. The insertions are not seen in wild-type revertants derived from these animals. Taken together these data provide strong evidence that .
mec-7 is the
tub-2 -tubulin gene. The predicted tubulin sequence has 441 amino acids. Our finding that
mec-7, a gene required for the production of 15-protofilament microtubules, codes for a -tubulin suggests that the constituent tubulins of the microtubule may affect the number of protofilaments. We have compared the deduced amino acid sequence of the
mec-7 -tubulin with those of other -tubulins to identify regions of the
mec-7 product that could potentially be required for polymerization into 15-protofilament microtubules. The
mec-7 tubulin is 90-93% identical to vertebrate -tubulins. The carboxyl-terminal region beyond amino acid 430 is highly divergent in
mec-7, as it is in all other -tubulins, and is the shortest of any reported -tubulin ( Sullivan and Cleveland, PNAS 83: 4327; Rudolph et al., Mol. Cell. Biol. 7: 2231; Burland et al., Mol. Cell. Biol. 8: 1275). In addition to the difference at the C-terminus, only seven residues in the
mec-7 peptide sequence are not shared by other reported -tubulins, including two other worm -tubulins (
tub-1 and
ben-1) these are residues 35 (gln), 127 (thr), 198 (ser), 278 (asn), 293 (cys), 343 ( asp), and 429 (ala). Two of these changes, those at positions 127 and 343, are in amino acids that have hitherto been absolutely conserved.