Characterization of the translational control mechanism preventing synthesis of alpha 2(I) collagen in chicken vertebral chondroblasts.

Chondrogenesis, the differentiation of mesenchyme into cartilage, involves a transition from synthesis of type I to type II collagen. Chicken vertebral chondroblasts contain both type II and alpha 2 type I collagen RNAs but synthesize only type II collagen, suggesting the existence of translational discrimination between these RNAs. The experiments outlined in this report examine the translational control mechanism preventing the synthesis of alpha 2(I) collagen in chondroblasts. Specifically, the alpha 2(I) collagen RNA in the cytoplasm of mature chondroblasts does not appear to be sequestered in ribonucleoprotein particles that could prevent its translation in these cells. Instead, the RNA associates with an average of only three ribosomes; each of these ribosomes appears to be capable of forming at least one peptide bond. However, treatment of chondroblasts with low concentrations of cycloheximide, an elongation inhibitor, suggests movement of the ribosomes on the alpha 2(I) collagen RNA may be partially blocked, resulting in a severe reduction in the translation elongation rate. This translational mechanism may constitute an important regulatory function mediating the cessation of type I collagen synthesis during chondrogenesis.