Abstract
Heat shock proteins (HSPs) are molecular chaperones that help organisms cope with stressful conditions. In this study, a novel small HSP (sHSP) gene was identified from the hard clam Meretrix meretrix (designated as Mm-HSP20). The full length of the cDNA is 1222 bp, con- sisting of a 5'-terminal untranslated region (5'UTR) of 103 bp, a 3'UTR of 565 bp, and a 522 bp open reading frame encoding a polypeptide of 173 amino acids. Sequence comparison showed that Mm-HSP20 had a moderate degree of homology to the sHSP of other organisms. A sHSP feature domain, an alpha-crystallin domain, and a V/IXI/V motif in the C-terminal extension were identified in the Mm-HSP20 amino acid sequence, indicating that Mm-HSP20 is a new member of the sHSP family. The Mm-HSP20 transcript was constitutively expressed in 6 different test tissues, with the highest expression level detected in the digestive gland by fluorescent quantitative real- time PCR. The temporal expression of Mm-HSP20 mRNA in the digestive gland and hemocytes was evaluated after exposure to cadmium (Cd) (40 µg l �1 CdCl2) and benzo(a)pyrene (50 µg l �1 B(a)P) individually, and in combination (40 µg l �1 CdCl2 +5 0 µg l �1 B(a)P). Mm-HSP20 expression level increased significantly after Cd and B(a)P exposure in both tissue types. Conversely, signifi- cant Mm-HSP20 transcript repression was detected in the 24 h B(a)P-treated digestive gland sam- ples. Considerable up-regulation of Mm-HSP20 mRNA level was observed after multiple expo- sures. These results indicate that Mm-HSP20 plays a role in mechanisms involved in coping with environmental stress in hard clams. Mm-HSP20 may be suitable for use as a biomarker for heavy metal and B(a)P contamination.
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