We identify DNA regions that are necessary for the ubiquitous expression of the Drosophila melanogaster α1-tubulin (α1t) gene. In vitro transcription showed that two upstream regions, tubulin element 1 (TE1 [29 bp]) and tubulin element 2 (TE2 [68 bp]), and a downstream region activate transcription. Germ line transformation demonstrated that these three regions are sufficient to direct the α1t core promoter to begin transcribing at the stage of cellular blastoderm formation and to continue thereafter at high levels in all tissues and developmental stages. Remarkably, mutation of any one of these regions results in high sensitivity to chromosomal position effects, producing different but reproducible tissue-specific patterns of expression in each transformed line. None of these regions behaves as an enhancer in a conventional germ line transformation test. These observations show that these three regions, two of which bind the GAGA transcription factor, act ubiquitously to insulate from position effects and to activate transcription. The results also provide vectors for ubiquitous expression of gene products and for examining silencer activities.
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