Synthesis of ribosomal proteins from stored mRNAs early in seed germination

In several eukaryotic organisms the mRNA expression for ribosomal proteins (RPs) is highly regulated at the translational level. The present work addresses the synthesis of RPs early in germination and particularly the origin of their transcripts. Excised maize axes were labeled with [35S]methionine for 6 h, and both cytoplasmic and ribosomal proteins were analyzed by electrophoresis and fluorography. It was found that some RPs are newly synthesized and already incorporated into ribosomes during this period. Synthesis of RPs was also observed under conditions of transcription inhibition, suggesting the presence of stored-RP transcripts in the embryonic axes. In vitro translation of polysomes isolated from alpha-amanitin-treated axes early in germination (3 and 9 h) was performed. Immunoprecipitation of the products revealed the synthesis of rps. Four heterologous probes two each of the small subunit (S4 and S6) and large (L3, L16) subunit rps were used to detect the correspondent transcripts within the stored mRNA stock of the embryonic axes. Both, by slot blot and northern analysis, a positive cross-reaction occurred for all the tested samples. Although only two of them (S4 and S6) seem to be stored as mature mRNA.