Induction of Uq activity and a mn-type mutant by 5-aza-2'-deoxycytidine

DNA modification, particularly methylation, has been found to be inversely correlated with gene activity (for review, see Doerfler, Ann. Rev. Biochem. 52:93-124, 1983). In maize, the loss of Mu1 activity is due to an increasing level of DNA methylation of the element (Chandler and Walbot, PNAS 83:1767-1771, 1986). Our finding on the activation of the Uq transposable element in somatic tissues of progeny kernels of four inbreds lacking Uq activity (MNL 60:5-6) has raised such a question as to whether the activated Uq comes from the ruq receptor element at the A locus or whether it is a methylated inactive Uq that becomes active.

One experimental approach to address this question is to use the deoxycytidine analog 5-aza-2'-deoxycytidine. The roots of germinating seedlings from 34 sectored BF2 progeny kernels of the 4 maize inbreds were dipped into a 30 uM 5-aza-2'-deoxycytidine solution for 72-90 hours, transferred into seed pots in the greenhouse, and grown for another 2 weeks before transplanting into the field. During this period, death of some root tips and folding of the leaves were common. Four seedlings died of the treatment, the rest were recovered and looked normal. These plants, together with 15 plants derived from the same source as a control, were crossed with a-ruq tester reciprocally. Examination of the progeny ears gave rise to the following results: One treated plant was totally sterile; another treated plant had one Uq element being activated that appeared to cosegregate with a mn type mutant gene, designed as mn-866248U. It was not transmissible via pollen:
 
  Standard Miniature  
Ear cl spotted 1-spot cl spotted 1-spot Total
866248U/a-ruq 129 0 3 3 143 1 279
a-ruq/866248U 322 0 6 0 0 0 328

A similar event of Uq activation happened in a single kernel at a frequency of about 6.1 X 10-6 in a separate experiment without any treatment. When the plant 866201X derived from this kernel of Uq spotting was crossed by an a-ruq tester, approximately 50% of the progeny kernels were miniature and Uq spotted and the other 50% were normally sized and not Uq spotted. However, tests are not available on its transmission through pollen. This mn mutant gene is named mn-866201X.

These two events are unrelated by immediate parental source and therefore represent two independent events.

Yong-Bao Pan and Peter A. Peterson


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