Targeting the MSL complex counteracts the effect of increased histone acetylation and does not induce dosage compensation. Lin Sun¹, Harvey Fernandez¹, Jilong Li², Jianlin Cheng², James Birchler¹. 1) Biological Sci Div; 2) Department of Computer Sci, Univ Missouri,Columbia, MO.

   In order to study the effect of histone modification produced by the histone acetyltransferase, MOF, a GAL4 DNA binding domain fusion was produced. Reporters were constructed that contained the GAL4 target sequences (UAS) preceding the mini-white reporter. In females there is a strong up-regulation of targeted mini-white insertions. Immunocytochemistry and ChIP demonstrated that the reporter had increased H4Lys16Ac thus showing a correlation between histone modification and gene expression. However, in males, the targeting of GAL4 MOF showed a reduced expression with all X and autosomal reporters. Interestingly, the autosomal reporter has the components of the MSL complex brought to the targeted reporter. For comparison, a GAL4-MSL2 fusion construct was made. When targeted to UAS-mini-white reporters, immunocytochemistry and ChIP showed that the components of the MSL complex were brought to the reporters and were effective in modifying H4Lys16. Using both molecular and phenotypic assays, there was no evidence that gene expression of the reporters was detectably changed suggesting that the MSL complex does not mediate dosage compensation. This hypothesis was further examined by conducting a global gene expression analysis of ectopically expressed MSL2 in adult females compared to normal females. Triplicate biological replicates were subjected to RNAseq and the average sequencing reads per gene were determined. The distribution of expression ratios of X chromosomal genes showed a major peak surrounding a value of 1.0 rather than 2.0 expected if compensation were induced. Phenotypic and northern validation using mini-white reporters on the X and the autosomes showed no increase in expression of the X reporter in females and a slight reduction of the autosomal reporter. The collective results are consistent with the hypothesis that the MSL complex overrides the effect of histone acetylation and is not the primary determinant of dosage compensation.