Transcriptional Control of δ-Crystallin Gene Expression in the Chicken Embryo Lens: Demonstration by a New Method for Measuring mRNA Metabolism

Abstract

Crystallins are proteins that accumulate to very high concentrations in the fiber cells of the lens of the eye. Crystallins are responsible for the transparency and high refractive index that are essential for lens function. In the chicken embryo, δ-crystallin accounts for more than 70% of the newly synthesized lens proteins. We used density labeling and gene-specific polymerase chain reaction (PCR) to determine the mechanism regulating the expression of the two very similar δ-crystallin genes. Newly synthesized RNA was separated from preexisting RNA by incubating the lenses with ¹⁵N- and ¹³C-labeled ribonucleosides and then separating newly synthesized, density-labeled RNA from the bulk of light RNA by equilibrium density centrifugation in NaI-KI gradients. The relative abundances of the two crystallin mRNAs in the separated fractions were then determined by PCR. This method permitted the quantitation of newly synthesized processed and unprocessed δ-crystallin mRNAs. Additional studies used intron- and gene-specific PCR primers to determine the relative expression of the two δ-crystallin genes in processed RNA and unprocessed RNA extracted from different regions of the embryonic lens. Results of these tests indicated that the differential expression of the δ-crystallin genes was regulated primarily at the level of transcription. This outcome was not expected on the basis of the results of previous studies, which used in vitro transcription and transfection methods to evaluate the relative strengths of δ-crystallin promoter and enhancer sequences. Our data suggest that the cultured cells used in these earlier studies may not have provided an accurate view of δ-crystallin regulation in the intact lens.