p1-vv85, a new maize variegated
pericarp allele, contains an insertion similar to the CACTA element, Mpi1
--Wang Y, Peterson, T
The maize p1 gene encodes a Myb-like factor that regulates transcription
of structural genes, including a1 and c2, required for red
phlobaphene pigmentation in certain floral organs, including kernel, pericarp,
and cob. We obtained a new p1 mutant allele designated p1-vv85,
from Dr. Peter A. Peterson, ISU. The p1-vv85 allele specifies variegated
pericarp and cob, and exhibits a very high rate of reversion to a state
giving red pericarp and cob. Genetic analysis suggested that the mutant
phenotype is not associated with Ac or En/Spm transposable
element systems (P.A. Peterson, personal communication). Genomic Southern
analysis indicated that p1-vv85 contains a 1.2 kb insertion in p1
gene intron 2, compared with its progenitor, P1-rr107B (from
the Maize Genetics Stock Center). This region was amplified by PCR and
found to contain a 1168bp insertion which has characteristics of the CACTA
transposable element family: the insertion is bounded by 3 bp target site
duplications, and it has 13bp TIR (terminal inverted repeats) which are
identical to that of the CACTA transposable element Mpi1 (Weydemann
et al., MNL 62: 48,1988). The insertion also has structured subterminal
regions that contain 12 copies and 6 copies of a 12bp motif at the 5�-
and 3�-ends of the fragment, respectively. Northern blot analysis revealed
that the p1 gene transcript level is greatly reduced in p1-vv85
pericarp, while a revertant allele contains a normal level of p1
transcripts. Surprisingly, Southern blot analysis detected no difference
in the structures of the p1-vv85 allele and two independent revertant
alleles. Specifically, no evidence of excision of the 1168 bp insertion
could be detected by either genomic Southern blot or PCR. Thus, the insertion
in p1-vv85 may regulate p1 gene expression by an epigenetic
mechanism. Analysis of DNA methylation of the p-vv85 mutant and
a revertant allele revealed some differences in methylation of the insertion
sequence, but not in the flanking p1 gene sequences. This result
is consistent with a model in which methylation of the insertion sequence
may affect the binding of a suppressor-like factor to the element. In summary,
the p1-vv85 allele contains a CACTA element insertion relative to
the progenitor P1-rr107B allele. However, the apparent "reversion"
of p1-vv85 to a functional P1-rr state does not result from
element excision, but rather by relief of suppression, which is correlated
with the methylation state of the CACTA element.
Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.
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