Ectopic expression of the Suppressor of Underreplication gene inhibits endocycles but not the mitotic cell cycle in Drosophila melanogaster

Abstract

The Suppressor of Underreplication ( SuUR) gene contributes to the regulation of DNA replication in regions of intercalary heterochromatin in salivary gland polytene chromosomes. In the SuUR mutant these regions complete replication earlier than in wild type and, as a consequence, undergo full polytenization. Here we describe the effects of ectopic expression of SuUR using the GAL4-UAS system. We demonstrate that ectopically expressed SuUR exerts qualitatively distinct influences on polyploid and diploid tissues. Ectopic expression of SuUR inhibits DNA replication in polytene salivary gland nuclei, and reduces the degree of amplification of chorion protein genes that occurs in the follicle cell lineage. Effects caused by ectopic SuUR in diploid tissues vary considerably; there is no obvious effect on eye formation, but apoptosis is observed in the wing disc, and wing shape is distorted. The effect of ectopic SuUR expression is enhanced by mutations in the genes E2F and mus209 ( PCNA). Differential responses of polyploid and diploid cells to ectopic SuUR may reflect differences in the mechanisms underlying mitotic cell cycles and endocycles.

Induced decondensation of heterochromatin inDrosophila melanogasterpolytene chromosomes under condition of ectopic expression of theSupressor of underreplicationgene

“…13 It is also well-documented that when SuUR gene dosage is increased, many more regions display pronounced underreplication; furthermore, upon strong SUUR overexpression via GAL4/UAS system from early embryogenesis, salivary glands remain tiny, which is indicative of suppression of DNA polytenization in these cells. 12,21 Taken together, these data are suggestive that SUUR overexpression in mid third instar larvae using an Sgs3-GAL4 driver which is manifested as swelling formation, should have a pronounced effect on a general DNA replication pattern in salivary gland cells. To test this, we made use of PCNA-specific antibodies, which label the replication machinery.…”

“…Such specific effects of SUUR overexpression are manifested as suppression of chorion gene cluster amplification and stalling of endocycles, which in turn results in a tiny salivary gland with small nuclei. 21 Likewise, swellings can be considered as a specific phenotype of SUUR overexpression, since most of the known chromatin proteins, such as POLYCOMB, SU(VAR)3-7, SU(VAR)3-9 and HP1 fail to produce similar chromatin changes. At the same time, this specific effect is distinct from DNA replication and DSB formation effects, since these effects segregate with different parts of SUUR protein.…”

Conservation of Domain Structure in a Fast-Evolving Heterochromatic SUUR Protein in Drosophilids

“…Ectopic expression of UAS-SuUR in follicular cells under the control of the C323-GAL4 driver suppresses amplification of chorion genes and results in complete female sterility (Volkova et al 2003). When SUUR Nmut was ectopically expressed under the C323-GAL4 driver, we observed weak suppression of the female sterile phenotype (20 crosses were set for 10 independent transgenic stocks, and in 2 crosses from different stocks single escapers were observed).…”

“…We examined the effects of ectopically expressed mutated proteins in the UAS-GAL4 system (Brand and Perrimon 1993). Permanent strong expression of SUUR in salivary glands under the AB1-GAL4 driver suppresses endoreplication and results in miniature salivary glands (Volkova et al 2003). Expression of SUUR Nmut under the control of AB1-GAL4 causes only partial suppression of endoreplication.…”

“…Conversely, an increase in the SuUR gene copy number enhances the underreplication in IH regions (Zhimulev et al 2003a;Belyakin et al 2005). Ectopic expression of SUUR in follicular cells suppresses the amplification of chorion gene clusters (Volkova et al 2003). Finally, strong SUUR overexpression in third instar larval salivary glands leads to structural changes (''swellings'') in chromosome morphology of PH and IH regions (Zhimulev et al 2003c).…”