In the last issue of MNL (59, p. 33, 1985) the sequence of the transposable element Activator was reported. Of the open reading frames, at least ORF1 is likely to encode a gene product, because a 194 bp deletion leads to loss of Ac activity and creation of the passive element Ds9 (Pohlman et al., Cell 37:635, 1984).

DNA segments of ORF1 therefore seemed most suitable for the construction of expression vectors with the aim of expression of an Ac-specific protein or a part of it in E. coli cells. The fragments were appropriately inserted into the vector pEx31 (kindly given to us by H. Schaller, Heidelberg); this allowed the production of a fusion polypeptide. These proteins have been purified from bacterial cultures, and antisera against the fusions were raised in rabbits.

Initial Western blotting experiments with total and nuclear extracts from endosperm or root tissue show several bands, of which only an approximately 90 - 100 kD band is consistently seen in extracts from Ac-containing plants; this band is absent in extracts from genetically Ac-strains. Another band corresponding to a protein of about 40 kD size lights up in all tissues. These bands are not detected with pre-immune serum.

Meanwhile RNA and cDNA data are available (see accomp. report by R. Kunze and P. Starlinger). The size of the large protein is in agreement with the open reading frame of the Ac cDNA (2421 bp = 807 aa). Whether the protein detected by these tests is indeed an Ac-encoded protein remains to be established.

M. Muller-Neumann, H. Fusswinkel and P. Starlinger

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