Mu elements are present in approximately
10-70 copies in Mutator lines of maize. They have been shown to reside
primarily in active regions of the genome and to exhibit a gene-specific
insertion preference (Alleman and Freeling, Genetics 112:107-119, 1986;
Bennetzen et al., UCLA Symp. Mol. Biol. 62:183-204, 1987). Thus, Mu
elements may serve as useful molecular markers for those active portions
of the genome in which they are found. We have used the internal MluI
fragment of the Mu1 element as a hybridization probe in Southern
analyses of DNA from Mutator embryogenic callus tissues. The six callus
lines examined were derived either from the F1's of H99/Mu32 crosses
or the self-pollinations of a bz-Mum8 plant. Each of these lines
contained approximately 20-40 Mu elements that were modified, as
determined by the failure of the restriction enzyme HinfI to cleave
the DNA at sites located within Mu-element inverted repeats. The
hybridization profiles of each line obtained by restriction digestion with
various enzymes that cut outside Mu1 and hybridization with the
Mu1 probe revealed restriction fragment uniformity and stability
over approximately one year in culture (10-60 weeks). This stability suggests
that gross rearrangements of regions of the genome occupied by Mu
elements did not occur in culture. Because Mu elements are associated
with active regions of DNA, stability of the active, or genic, portion
of the genome during culture is implied.
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