Bovine opsin gene expression exhibits a late fetal to adult regulatory switch

Rates of bovine photoreceptor gene transcription, as measured by nuclear run‐on assays, exhibit gene‐specific patterns of regulation. Here we investigate initiation and elongation in nuclear run‐on assays with the use of sarkosyl to further understand the nature of these gene‐specific elements. Opsin transcription, alone among several genes tested, proved sarkosyl‐sensitive. This sensitivity is maximal in adult retinas, with inhibition first detected in mid‐third trimester fetal retinas. Therefore, opsin transcription appears to involve different regulatory elements in adult and fetal retinas, implying a fetal to adult switch in the control of opsin gene expression. Although this regulatory switch is initially activated at a time when the fetal outer nuclear layer of the retina first achieves adult‐like morphology, further maturation of opsin regulation takes place postpartum since levels of sarkosyl sensitivity are almost 5‐fold greater in adult retinas compared to the 7.5 month fetus. We also show that the sarkosyl‐induced reduction of opsin transcripton is not due to prevention of de novo RNA polymerase II initiation in the run‐on reaction, suggesting the detergent alters a positive‐acting, postinitiation component of the transcriptional apparatus. Since levels of opsin transcription with sarkosyl are similar to those of the other visual transduction genes with or without sarkosyl, this detergent‐sensitive transcriptional component appears to account for the singularly high, gene‐specific rate of opsin transcription in retinal photoreceptor cells. © 1995 Wiley‐Liss, Inc.