Developmental time-course of gene inactivation caused by position effect. Aleksei Shatskikh, Sergey Lavrov, Vladimir Gvozdev. Department of Molecular Genetics of Cell, Institute of Molecular Genetics of Russian Academy of Sciences, Moscow, Russian Federation.

   In(2)A4 is a chromosomal rearrangement with breakpoints in 39A region and pericentromeric heterochromatin of chromosome 2L. This inversion causes position effect variegation in euchromatic region that is adjacent to the breakpoint. In In(2)A4, expression of genes is perturbed at distances of approx. 40-50 kb starting from eu-heterochromatin boundary. In adults no continuous spreading of inactivation is observed and many genes escape inactivation. We suggested that earlier in ontogenesis some genes within the affected region are prone to inactivation. Expression of affected genes was measured in larvae and pupae. We detected paradoxical effect of PEV-induced activation of some genes in larvae. Interestingly, at different stages of development heterochromatin may disturb the expression of the same gene in the opposite direction. The results of time-course mRNA level measurements suggest that heterochromatin mostly affects expression of a target gene at the stage of its transcriptional activation in ontogenesis. These data may give a clue to mechanisms of interaction between heterochromatinization and transcriptional machinery.