Repetitive DNA sequences in maize: Possible roles in plant adaptation and transient gene expression

Fluctuations as high as 40% in the nuclear DNA content in maize have been observed as a result of adaptive response. The knob DNA sequence has been reported to contribute 16% of this variation. Other DNA sequences involved in genome size variation due to adaptive response have yet to be identified. To isolate and identify such sequences, genomic DNA libraries were constructed from three maize lines adapted to different altitudes. The isolated clones were screened for high copy number inserts through hybridization studies. The relative abundance of six selected clones was determined in 10 maize lines adapted to altitudes ranging from 100-7000 feet. Statistical analysis indicated that two of the isolated sequences had significant positive correlation between altitude and relative abundance in Arizona Indian maize lines adapted to 100-4000 feet. Four of the clones indicated variation in abundance independent of the altitude in all the tested maize lines. These observations provide evidence for the role of repetitive DNA sequences, other than knob, in adaptation of maize to different environmental conditions.;One of the moderately abundant maize genomic DNA segment in clone PI44-98 was found to be equally abundant in various maize inbred lines capable of tissue culture regeneration. The nucleotide sequence of this fragment was determined and homology search was performed. The 662 bp fragment contained sequences similar to a Petunia genomic DNA sequence and to a copia like transposable element in potato. This sequence also contained homology to ARS sequences found in maize. The 662 bp sequence was used to flank the hygromycin phosphotransferase and UidA genes in plasmid pBlueScript in a direct repeat order. The resulting construct was used in both circular and linear forms to transform type 1 embryogenic calli of the maize line H99 through particle bombardment. The statistical analysis of the transient expression frequencies of UidA gene indicated that the dispersed repeated sequence used in this study increased transient expression efficiency by 17% as the circular form and by 50% in the linear form. The results indicate that maize repetitive DNA sequences may be used to enhance the transient expression of foreign genes in maize.