In the ongoing saga of the
hsp70 multigene family, we have recently sequenced the
hsp70B gene. It has been suggested that the
hsp70B gene is a pseudogene of the constitutively expressed, heat inducible
hsp70A gene (Snutch, Heschl and Baillie, manuscript submitted). This was based on the ability of the
hsp70B gene to cross-hybridize with the
hsp70A gene under conditions of high stringency. Sequence analysis of the
hsp70B gene and a comparison to the
hsp70A gene show that: 1) the homology starts within the 5' untranslated leader sequence but does not include the 5' regulatory region, 2) only two of the introns and their positions are conserved, 3) the
hsp70B sequence lacks the last 40% of the corresponding
hsp70A sequence including the last intron, 4) there are two stop codons within the
hsp70 reading frame, 5) there are several single base pair deletions and one insertion resulting in frameshifts. The one insertion is compensated for by a deletion 3 bp downstream, 6) there is an internal deletion of 243 bp, 7) the nucleotide substitution rate at the third, second and first codon positions are 12.1%, 5.2% and 7.2% respectively, and 8) there is a greater number of deletions and insertions occurring within the introns than within the 'coding region' of
hsp70B. Features #3 to #6 support the idea that the
hsp70B gene is a pseudogene of
hsp70A. Features #7 and #8 suggest that the
hsp70B gene was functional for a while after the
hsp70A gene duplication event ( third codon position substitution rate is almost double that of the first and second positions. If the gene was inactive from the point of the duplication event, the number and types of mutations would have been the same regardless of the codon position or position within the intron or the coding region.) We are currently attempting to calculate when the
hsp70A gene was duplicated. A putative TATA box was found in a position similar to that of the
hsp70A gene although the surrounding nucleotide sequence is very different. In addition, there are two possible poly A addition sites (with one base pair differences) downstream of the
hsp70B/hsp70A homology. It will be interesting to see if this pseudogene, which is not present in the sister species C. briggseae, is present in any, or all, of the other wildtype strains of C. elegans. This information could tell us when the different wildtype strains diverged.