The protein product of the wx locus has been identified and isolated. When starch granules from Wx endosperm are extracted by heating in SDS containing buffer and run on SDS polyacrylamide gels, five proteins are visualized by Coomassie Blue staining. One major protein is present which makes up about 85% of the total extracted proteins and has a molecular weight of approximately 60,000. The four other minor proteins are all of higher molecular weights. When the starch granules from endosperms of 29 wx alleles which produce no amylose were examined, 25 showed a complete lack of the major protein, termed the Wx protein, while the minor proteins were unaffected. This difference between Wx and wx endosperm was observed from 12 days to maturity. Also, starch granules from Wx pollen have the Wx protein while those from wx pollen do not. The other four alleles, B3 (an Mp controlling element mutation), C31, R and 90 all make the Wx protein. B3 and C31 show normal levels of the protein and R and 90 show reduced levels. All alleles which produce reduced amounts of amylose have some Wx protein. m-8, an Spm mutation, has less than 3% the normal level, wx-a has about 5%, and S5 and S15, two stable partial revertants of the Ds mutant m-1, obtained from 0. Nelson, have full levels.
That the Wx protein is coded for by the wx locus is demonstrated by the facts that the Wx protein varies linearly with Wx allele dosage and that Wx proteins with altered isoelectric points are produced by the wx alleles 90, C31 and R. Our results indicate that the Wx protein is the starch granule-bound NDP sugar-starch glucosyl transferase since this enzyme activity is also absent in wx endosperm (Nelson and Rines, Biochem. Biophys. Res. Comm. 9:297) and varies linearly with Wx dosage (Tsai, MGNL 39:153).
The data also show that wx is the locus of the structural gene. Since
altered proteins are produced by alleles mapping near both ends of the
locus (C31, R and 90) the structural gene extends throughout most of the
mapped region (Nelson, MGNL 50:109). Thus, the controlling element mutations
B3 (Mp), m-1 (Ds) and m-6 (Ds), and probably also B4 (Ds) and m-8 (Spm)
lie within the structural gene. It may be that some of them are in an internal
control region (regulatory introns) but this certainly is not the case
for B3, which produces the full level of an inactive transferase. The same
is true for m-1 which, after presumed transposition of the Ds element away
from the locus, gives rise to two fully expressed but only partially active
Wx proteins, those specified by S5 and S15. No size differences have been
detected on SDS gels among any of the allelic Wx proteins. Our analysis
indicates some form of RNA processing for B3 and m-8.
|Allele||Amylose content||% Wx protein (relative to Wx)||Controlling element|
All of the other wx mutants mapped by Nelson, including the unmapped alleles I, P60 and BL3, lack the Wx protein. Alleles J and K were not tested.
Craig Echt and Drew Schwartz
Return to the MNL 55 On-Line Index
Return to the Maize Newsletter Index
Return to the Maize Genome Database Page