Characterization of a salivary lysozyme in larval Helicoverpa zea

The cDNA sequence of a salivary lysozyme in Helicoverpa zea (Lepidoptera: Noctuidae) was determined. The full-length cDNA is 1,032 bp, and it encodes a protein of 142 amino acids. This lysozyme has 90% identity with Heliothis virescens lysozyme and 76% identity with Manduca sexta lysozyme. There is a signal peptide of 20 amino acids at the N-terminus. The mature protein is about 14.4 kDa without the signal peptide. The pI value is greater than 9.5 as determined by isoelectric focusing. From genomic DNA, two introns and three exons were within the open reading frame (ORF). Southern blot analysis indicated that it is a single-copy gene. A time-course study revealed that the H. zea lysozyme gene was differentially expressed in the labial glands during the development of fifth-instar larvae, with the peak level of lysozyme mRNA being detected on day 1. Dot blot analysis showed different levels of H. zea lysozyme expression when the caterpillars fed on different plants. Further, the H. zea lysozyme could be detected with antibodies raised against the M. sexta lysozyme, and it was one of the most abundant secreted proteins in saliva collected directly from the caterpillar's spinneret. The potential role of the lysozyme on host plants in mediating susceptibility to bacterial disease is discussed in the context of tritrophic interactions.