The BmChi-h gene, a bacterial-type chitinase gene of Bombyx mori, encodes a functional exochitinase that plays a role in the chitin degradation during the molting process.
The silkworm, Bombyx mori, has been recently demonstrated to contain a bacterial-type chitinase gene (BmChi-h) in addition to a well-characterized endochitinase gene (BmChitinase). The deduced amino acid sequence of BmChi-h showed extensive structural similarities with chitinases from bacteria such as Serratia marcescens chiA and baculoviruses (v-CHIA). Bacterial-type chitinase genes have not been found from any eukaryotes and viruses except for lepidopteran insects and lepidopteran baculoviruses. Thus, it was suggested that BmChi-h may be derived from a bacterial or baculovirus chitinase gene via horizontal gene transfer. In this report, we investigated the biological function of BmChi-h. Our enzymological study indicated that a chitinase encoded by BmChi-h has exo-type substrate preference, which is the same as S. marcescens chiA and v-CHIA, and different from BmChitinase, which has endo-type substrate preference. An immunohistochemical study revealed that BmChi-h localizes in the chitin-containing tissues during the molting stages, indicating that it plays a role in chitin degradation during molting. These results suggest that BmChi-h (exochitinase) and BmChitinase (endochitinase) may catalyze a native chitin by a concerted mechanism. Cloning and comparison of BmChi-h orthologues revealed that bacterial-type chitinase genes are highly conserved among lepidopteran insects, suggesting that the utilization of a bacterial-type chitinase during the molting process may be a general feature of lepidopteran insects.
Daimon
Takaaki
T
Department of Agricultural and Environmental Biology, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8657, Japan.
Katsuma
Susumu
S
Iwanaga
Masashi
M
Kang
WonKyung
W
Shimada
Toru
T
D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D000818:Animals; D001483:Base Sequence; D012831:Bombyx / Q000201:enzymology / Q000235:genetics* / Q000254:growth & development; D002686:Chitin / Q000378:metabolism*; D002688:Chitinases / Q000235:genetics*; D017124:Conserved Sequence; D017931:DNA Primers; D006863:Hydrogen-Ion Concentration; D008969:Molecular Sequence Data; D018978:Molting / Q000502:physiology*; D016297:Mutagenesis, Site-Directed; D016133:Polymerase Chain Reaction; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid
D017931:DNA Primers; D002686:Chitin; D002688:Chitinases