CADMIUM-INDUCED EXPRESSION OF METALLOTHIONEIN AND SUPPRESSION OF RNA TO DNA RATIOS DURING MOLLUSCAN DEVELOPMENT

One-week-old larvae of the mollusc Crassostrea virginica, an oyster, w ere exposed to Cd concentrations ranging from the control with no adde d Cd to 0.2 mu M CdCl2 for 24 hr. Concentration-dependent increases in total cadmium accumulation, cytosolic and metallothionein-bound Cd co ncentrations, and levels of the MT mRNA were detectable over this rang e of exposure concentrations. Increases in these measures were apparen t at the lowest exposure concentration of 0.005 mu M, With dramatic in creases observed between 0.04 and 0.2 mu M Cd. Although the concentrat ions of MT-bound Cd increased, the fraction of total Cd that was bound to MTs declined in larvae treated with 0.04 and 0.2 mu M Cd, presumab ly due to the inability of MT biosynthesis to keep up with the higher rates of Cd uptake at the higher concentrations. This decreased effect iveness of MTs in sequestering Cd in relation to the total Cd load and the associated increase in the nonthionein Cd fraction at these highe r treatment conditions coincided with a decline in the RNA to DNA rati o and an increase in a nonacetylated variant of the MT. The former is indicative of an inhibition of larval metabolism and growth. The latte r is an unexpected form for this MT and is believed to signify disrupt ion of normal cotranslational modification of the N-terminal amino aci d sequence. (C) 1995 Academic Press, Inc.