Post-transcriptional mechanisms control XXXX synthesis during its light-induced turnover in rye leaves through the availability of the hemin cofactor and reversible changes of the translation efficiency of mRNA.

@entity2196 @entity8618
The enzyme @entity2196 is light-sensitive. In leaves, losses caused by photoinactivation are replaced by new enzyme and the rate of de novo synthesis must be rapidly and flexibly attuned to fluctuating light conditions. In mature rye leaves, post-transcriptional mechanisms were shown to control the rate of @entity2196 synthesis. The amount of the leaf @entity2196 ( @entity8618 ) transcript did not increase with light intensity, but was even higher after dark exposure of light-grown leaves. Initiation was apparently not limiting translation in the dark, as the association of the @entity8618 mRNA with polysomes did not change notably under different light conditions. By analysing the translation of @entity2196 polypeptides in cell-free systems with poly(A)+ RNA from leaves or with mRNA transcribed from a @entity8618 -containing cDNA clone, two mechanisms of post-transcriptional control were identified. First, translation of @entity2196 depended on the presence of hemin. In leaves, the availability of hemin may signal the extent of @entity2196 degradation as the hemin of the inactivated enzyme is recycled. Second, the translation efficiency of the @entity8618 transcripts was reversibly modulated in a dose-dependent manner by the light intensity to which leaves were exposed, prior to extraction. The @entity8618 mRNA from light-exposed leaves was translated much more efficiently than mRNA from dark-exposed leaves. The increase of its translation activity in vivo was not blocked by cordycepin but was suppressed by methylation inhibitors, indicating a reversible modification of pre-existing mRNA by methylation. Translation of in vitro synthesized @entity8618 mRNA required a methylated cap (m7GpppG), but was virtually below detection when it contained an unmethylated cap (GpppG).
