Abstract
Two models of dosage compensation have been tested by the measurement of G6PD and 6PGD enzymatic specific activities in flies hyperploid for regions of the X chromosome. Females duplicated for the proximal half of the X chromosomes (2 1/2 X's) have an increased level of G6PD and a normal level of 6PGD. Females duplicated for the distal half of the X chromosome (2 1/2 X's) have a normal level of G6PD and an increased level of 6PGD. Males bearing duplications of various segments of the X chromosome show control levels of G6PD and 6PGD, except where the duplicated region includes the structural gene for G6PD or 6PGD. These results fail to provide evidence for either the presence of discrete X-linked compensator (regulator) genes reducing the activity of other X-linked genes, or for a factor in limiting supply necessary for the transcription of all the genes on the X chromosome. Superfemales (3 X chromosomes) have the same G6PD and 6PGD activity levels as their diploid sisters. It would appear that the regulation of gene activity by dosage compensation is a chromosomal phenomenon in that the level of activity per gene copy for loci on the X chromosome is modulated in a stepwise fashion according to the total number of X chromosomes present.
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