THE PROTEINS ENCODED BY 2 TAPETUM-SPECIFIC TRANSCRIPTS, SATAP35 AND SATAP44, FROM SINAPIS-ALBA L ARE LOCALIZED IN THE EXINE CELL-WALL LAYEROF DEVELOPING MICROSPORES

By differential screening of a copy DNA (cDNA) library from flowering Sinapis alba L. apices against cDNAs from vegetative apices, two cDNA clones were isolated representing transcripts that are expressed trans iently at an early stage of tapetum development. The Satap35 cDNA enco des a polypeptide with a predicted molecular weight of 12.7 kDa and an isoelectric point of 10.4. The Satap44 cDNA codes for a putative 12.4 -kDa polypeptide with an isoelectric point of 7.5. The deduced amino-a cid sequences display 76% sequence identity and contain an N-terminal stretch of hydrophobic amino acids which has characteristics of secret ory signal sequences. In-vitro transcription of the cDNAs and translat ion of the resulting RNAs in the presence of canine pancreatic microso mes demonstrates that the two proteins are translocated into the micro somes and that the putative preproteins are proteolytically processed to the mature forms. By immunoelectron microscopy the SaTAP35 and SaTA P44 proteins were detected at the developing peritapetal membrane betw een the tapetal cytoplasm and the adjacent middle layer of the anther wall. Furthermore, labelling was observed within the locule in associa tion with globules resembling pro-Ubisch bodies which appeared at the tetrad stage. During the early vacuolate stage of microspore developme nt the young exine was strongly labelled. The exine and the peritapeta l membrane both are composed of sporopollenin, and the pro-Ubisch bodi es are thought to contain sporopollenin precursors. Thus, SaTAP35 and SaTAP44 might be involved in sporopollenin formation and/or deposition .