Trans-inactivation: Repression in a wrong place

Abstract

Trans-inactivation is the repression of genes on a normal chromosome under the influence of a rearranged homologous chromosome demonstrating the position effect variegation (PEV). This phenomenon was studied in detail on the example of brown^Dominant allele causing the repression of wild-type brown gene on the opposite chromosome. We have investigated another trans-inactivation-inducing chromosome rearrangement, In(2)A4 inversion. In both cases, brown^Dominant and In(2)A4, the repression seems to be the result of dragging of the euchromatic region of the normal chromosome into the heterochromatic environment. It was found that cis-inactivation (classical PEV) and trans-inactivation show different patterns of distribution along the chromosome and respond differently to PEV modifying genes. It appears that the causative mechanism of trans-inactivation is de novo heterochromatin assembly on euchromatic sequences dragged into the heterochromatic nuclear compartment. Trans-inactivation turns out to be the result of a combination of heterochromatin-induced position effect and the somatic interphase chromosome pairing that is widespread in Diptera.

Keywords: PEV; chromatin; heterochromatin; nuclear compartments; trans-inactivation; transcription.

Figure 1.

The possible configuration of the loop between In(2)A4 inversion and the wild type chromosome relative to the euchromatic and heterochromatic nuclear compartments in interphase nuclei. On the top of the drawing, the linear map of the trans-inactivation distribution in the wild-type chromosome (dodger blue) paired with In(2)A4 (red) is shown. Blue triangles mark the position of non-inactivated transgenes, orange ones – trans-inactivated, the position of the histone genes cluster is shown in green, and pericentromeric heterochromatin is gray boxes. The vertical dotted line represents the In(2)A4 breakpoint location (39B). Checked transgenes could be grouped into 7 areas marked A, B, C, D, E, F and G, where A and G represent the regions outside the trans-inactivation spreading zone, B and C are the regions of approximately 50 kb in size with complete repression, D is the region where some transgenes are inactivated while the others are not. E is the region near the histone genes cluster, no repression observed here, and F is the “island” of trans-inactivation after the histone genes cluster. Below the linear map, the putative organization of the loop of paired chromosomes in the nuclear space is shown according to a combination of FISH data and polytene chromosomes arrangement. The light green is euchromatin; the brown gradient zone is HP1a-stained pericentromeric heterochromatin compartment. The green area is the histone genes cluster (HLB – histones locus body), which was shown to locate on the border of HP1a-enriched volume . The positions of the areas of trans-inactivation spreading (A, B, C, D, E, F and G) correspond to those on the linear map. According to the model, the regions near the breakpoint (B and C) are dragged deepest into heterochromatin and fully repressed, while the region near the histone genes cluster (E) is transferred outside the heterochromatin due to the specific properties of HLB, and the transgenes in this region are not inactivated.

Figure 2.

Two models of trans-inactivation establishment. A. Heterochromatin proteins and the histone modifications form a self-assembled complex propagating in cis from the new eu-heterochromatin border deep into the euchromatin. Upon reaching the trans-inactivated reporter gene position, the heterochromatin complexes either jump to paired chromosome or interact with regulatory elements of a reporter gene (like in transvection), causing the repression. B. The reporter gene is dragged into the heterochromatin due to pairing with the rearranged chromosome. The high local concentration of heterochromatin components like HP1a facilitates de novo formation of the heterochromatin on the reporter gene sequence. The peculiarities of In(2)A4-caused trans-inactivation (a vast distance of spreading, the lack of correlation between cis- and trans-inactivation, HP1a binding to reporter transgene but not to the same region on the opposite rearranged chromosome) favor the model B of the trans-inactivation establishment.