Institut für Genetik

The synthesis or activation of a trans-acting factor is not the rate-limiting step in excision/transposition events

--Manfred Heinlein and Peter Starlinger

Barbara McClintock has observed a correlation between the excision of Ds from the c-m1 allele and chromosome breakage at the site of Ds at standard position. If this would be a general effect, it might indicate that the activation or synthesis of a trans-acting factor is the limiting step in the transposition process. However, on the basis of our revised measurements of Ac-mRNA (Schein et al., MNL 64, 1990; Fusswinkel et al., Mol. Gen. Genet., in press), which yield 2 to 13 mRNA molecules per endosperm cell on the average, a limitation of transposition by the availability of Ac protein is not expected. Thus, the correlation of excision events was studied further (Heinlein and Starlinger, Maydica, submitted). We examined kernels on which the variegation patterns of two mutable alleles can be monitored. A lack of correlation is only diagnostic if all excision events lead to a visible phenotype. Due to excision “footprints” and other complications, we could not be sure to find such pairs of mutants. Therefore we looked for situations in which the correlation of events does not exist. This is the case when small sectors due to late events at the first mutable allele are located within larger sectors due to earlier events at the second locus. The small sectors are part of the larger sectors, meaning that the two events happened at different times but in the same cell clone. We observed such situations in several cases. Therefore we think that excision events are usually not correlated and are independent from each other. Thus, the low frequency of excision events cannot be explained by the lack of a trans-acting factor. B. McClintock's observation of correlated excision needs an additional explanation.

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