Structure of a tobacco endochitinase gene: evidence that different chitinase genes can arise by transposition of sequences encoding a cysteine-rich domain.

The endochitinases (E.C. 3.2.1.14, chitinase) are a structurally diverse group of enzymes believed to be important in the biochemical defense of plants against potential pathogens. The gene for a chitinase of Nicotiana tabacum L. cv. Havana 425 has been cloned and sequenced. The major transcription start is 11 bp upstream of the ATG codon and 28 bp downstream of the TATA box. The gene contains two introns and encodes a basic chitinase of 329 amino acids with a 23 amino acid N-terminal signal peptide followed by a 43 amino acid, cysteine-rich domain, which is linked by a hinge region to the main structure of the enzyme. This gene appears to be expressed because the exons are identical to the coding sequence of a cDNA which was isolated. Comparison of chitinase amino acid sequences from different plants indicates there are at least three classes of these enzymes: class I, basic chitinases with an N-terminal cysteine-rich domain and a highly conserved main structure; class II, chitinases similar to the main structure of class I chitinases but lacking the cysteine-rich domain; and, class III, chitinases with conserved sequences different from those of the class I and II enzymes. The sequences encoding the cysteine-rich domain in class I chitinases are flanked by 9-10 bp imperfect direct repeats suggesting that these domains arose from a common ancestral gene and were introduced into genes for class I enzymes by transposition events.