AB 18:243-252 (2013)  -  DOI: https://doi.org/10.3354/ab00503

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, consisting 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, significant Mm-HSP20 transcript repression was detected in the 24 h B[a]P-treated digestive gland samples. Considerable up-regulation of Mm-HSP20 mRNA level was observed after multiple exposures. 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.

KEY WORDS: Hard clam · Meretrix meretrix · HSP20 · mRNA expression · Heavy metal · Cd · Benzo[a]pyrene · Biomarker