Autoregulatory control of translatable phytochrome mRNA levels

Abstract

Translatable phytochrome mRNA represents .apprxeq. 5 .times. 10-3% of the total poly(A)-RNA present in etiolated Avena sativa L. cv. Garry seedlings, as determined by incorporation of radioactivity into the immunoprecipitable apoprotein in a cell-free translation system. Irradiation of such seedlings with 5-s red light induces a decline in this mRNA that is detectable within 15-30 min, shows a 50% reduction within 50-60 min, and results in a > 95% reduction within 2 h. The effect of the red light pulse is reversed by an immediately subsequent far-red pulse to the level of the far-red-light control, indicating that phytochrome exerts autoregulatory control over its own translatable mRNA level. This result necessitates revision of existing concepts of how phytochrome concentrations are modulated in vivo. Red-light dose-response curves show that the response is sensitive to very low light levels. Conversion of < 1% of the total cellular phytochrome to the biologically active far-red-absorbing form is sufficient to induce .apprxeq. 60% of the maximal response, and 20% far-red-absorbing form saturates the response. The observed change in translatable phytochrome mRNA level is one of the most rapid phytochrome-induced alterations in any cellular mRNA yet recorded. Thus, autoregulation of phytochrome mRNA provides an attractive opportunity to examine the early sequence of events in phytochrome control of gene expression.