In vitro reconstitution of legumin (11S) mRNA and binding proteins as related to post-transcriptional regulation of protein synthesis in developing alfalfa embryos

Abstract

There is undetectable transcription of 11S storage protein (medicagin) mRNA by nuclei isolated from pre-cotyledonary-stage somatic embryos of alfalfa (Medicago sativa L). However, this message exists at steady-state levels in the embryos at this stage of development without concomitant synthesis of the storage protein. At the pre-cotyledonary stage, therefore, the transcriptional rate for 11S mRNA is low; what message is transcribed is sequestered in the form of mRNP complexes and is not recruited into polysomes in vivo (33). Both transcription (in vivo and in isolated nuclei) and translation of the 11S mRNA are evident at the onset of cotyledon development in somatic and zygotic embryos, reaching a maximum during expansion of the cotyledons and then declining as the embryos mature. Pre-cotyledonary-stage somatic embryos which do not utilize the 11S-mRNA in polysomes lack certain mRNA-binding proteins (32, 36 and 38 kD) which are present at later stages of development. These mRNA-binding proteins may be responsible for the initiation of large polysome formation since they were exclusively present in the translational extracts of cotyledonary somatic and zygotic embryos in which there was no repression of storage protein synthesis. In contrast, the pre-cotyledonary somatic embryos contained a different set of 11S-mRNA-binding proteins (28, 50, 55, and 62 kD) whose presence in the cotyledonary stage embryos was very rare or non-existent; these could be responsible for preventing translation.