THE INVOLUCRIN GENE OF THE TREE SHREW - RECENT REPEAT ADDITIONS AND THE RELOCATION OF CYSTEINE CODONS

The coding region of the involucrin gene of Tupaia glis has been clone d and sequenced. It resembles the involucrin coding region of other no n-anthropoid mammals in possessing a segment of related, short tandem repeats at a defined location, but in Tupaia, there has been recent se rial duplication of a repeat into which a cysteine codon had earlier b een introduced. As a result of the duplication, there is a total of as many as six cysteine codons in the segment of repeats, a number large r than for any other species yet examined. In Rattus there has been a comparable but independent addition of cysteine codons, and both Tupai a and Rattus have eliminated an otherwise conserved cysteine codon 75 located close to but outside the segment of repeats. In Tupaia, this e limination probably occurred by gene conversion. Also independently, t he gene of Canis has added cysteine codons to the segment of repeats b ut has not yet lost cysteine 75. It is proposed that the gain and the loss of cysteine codons are parts of a multi-stage program of cysteine relocation.