chemical causing callus formation, suggesting that it may be A basic chitinase was secreted into culture medium of pumpkin cell suspension cultures. The chitinase was purified from independent of the presence of 2,4-D. Perhaps, induction is caused by osmotic or wounding stress. Levels of chitinase the culture medium. A cDNA encoding the pumpkin chitinase mRNA markedly increased at 4 h after transfer of pumpkin was cloned by reverse transcription (RT)-PCR and rapid amplification of cDNA ends (RACE) methods. The chitinase callus cells into fresh culture liquid medium. They were also high at later stages of cell suspension culture. In transgenic gene was strongly expressed in pumpkin callus cells, but little or not at all in mature leaf, young leaf, cotyledon, stem, tobacco BY-2 cells, into which the pumpkin chitinase cDNA hypocotyl and root of pumpkin. No chitinase mRNA was was introduced, the recombinant pumpkin chitinase was expressed and secreted into the culture medium, suggesting that detected in intact pumpkin fruit tissues. However, chitinase was induced during callus formation from sliced pumpkin fruit the signal peptide of pumpkin chitinase also functions for secretion from tobacco BY-2 cells. tissues. Induction also occurred in the absence of 2,4-D, a perhaps by releasing endogenous factors that are related to nodulation factors (De Jong et al. 1992, Vijn et al. 1993.In plants, four classes of chitinases have so far been proposed on the basis of their primary structures (Collinge et al. 1993). Class I chitinases contain an amino-terminal cysteine-rich domain of about 40 amino acids, a chitinbinding domain that is homologous to hevein and wheat germ agglutinin, and a highly conserved main structure, separated by a variable hinge region. Most class I chitinases have a basic isoelectric point and are localized in vacuoles. Indeed, many prochitinases have vacuolar targeting signals, i.e seven carboxy-terminal extra amino acids (Neuhaus et al. 1991). Class II chitinases are similar to class I chitinases but lack an amino-terminal cysteine-rich domain. They appear to be acidic proteins and are located in the apoplast. Class III chitinases have sequences very different from most of class I and class II chitinases. They can be acidic or basic proteins, and may be localized in the apoplast. Class IV chitinases contain a cysteine-rich domain and a conserved main structure, resembling class I chitinases, but they are significantly smaller as a consequence of four major deletions.
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