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The transparent cellular eye lens of the jellyfish (Tripedalia cystophora) contains three major proteins called J1-, J2-, and J3-crystallins. Here we have isolated cDNAs encoding three novel 37-kDa J1-crystallin polypeptides (J1A, J1B, and J1C) sharing 84-98% identity in amino acid sequence among themselves. Each polypeptide is encoded in a separate gene lacking introns. In contrast to the striking similarity of the coding regions, the 5'- and 3'-untranslated sequences of the three J1-crystallin mRNAs are completely different, consistent with an ancient duplication of their genes. Thermostability experiments showed that J1-crystallins remain soluble at 50 degrees C, but precipitate at 60 degrees C, suggesting that these major lens proteins are neither heat shock proteins nor unusually heat-resistant as are many vertebrate crystallins. Although J1 mRNAs appear polyadenylated, no typical polyadenylation signal was detected in the cDNAs. Surprisingly, the only obvious similarities among the 5'-flanking regions of the three J1-crystallin genes are putative TATA boxes and several CAAT sequences, consistent with fewer evolutionary constraints on the regulatory sequences than on the coding sequences of these crystallin genes.
Piatigorsky et al. (Tue,) studied this question.