In examining somatic instability in the early excision bz2-mu2
line described in an accompanying note, I found null sectors in addition
to purple sectors. In material with more than one dose of the reporter
allele, null sectors are very likely to represent loss of Mutator activity.
In outcross ears with just one dose of the reporter allele, however, null
sectors could represent either loss of activity or excision events that
fail to restore sufficient Bz2 activity to condition a purple phenotype.
Because Mu somatic excision is not very precise, usually resulting
in small deletions (A. B. Britt, unpublished data), only one-third of Mu
excisions from an exon are expected to restore the reading frame. Seven
ears -- outcross progeny of 3 different plants -- were analyzed, and null
sectors were found to be about 3 times more common than purple sectors
of the same size.
|Sector Size||Purple Sectors||Null Sectors|
The excess of null sectors compared to predictions based solely on restoration
of the reading frame (Exp. column) could result from  loss of Mutator
activity (fewer than expected spotted kernels were recovered in this population),
and  deletions or alterations that mutate an essential gene element,
i.e. loss of the initiator or termination codon, loss of a site required
for splicing, or amino acid changes that destroy protein function. The
precise location of the Mu1 insertion in bz2-mu2 is not known
but by Southern mapping it is in the reading frame near the ATG. Alterations
of the reading frame would, therefore, be expected to yield null function.
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