Su(UR)ES: A gene suppressing DNA underreplication in intercalary and pericentric heterochromatin ofDrosophila melanogasterpolytene chromosomes

Abstract

A genetic locus suppressing DNA underreplication in intercalary heterochromatin (IH) and pericentric heterochromatin (PH) of the polytene chromosomes of Drosophila melanogaster salivary glands, has been described. Found in the In(1)sc V2 strain, the mutation, designated as Su(UR)ES, was located on chromosome 3L at position 34.8 and cytologically mapped to region 68A3-B4. A cytological phenotype was observed in the salivary gland chromosomes of larvae homozygous and hemizygous for Su(UR)ES: (i) in the IH regions, that normally are incompletely polytenized and so they often break to form ''weak points,'' underreplication is suppressed, breaks and ectopic contacts disappear; (ii) the degree of polytenization in PH grows higher. That is why the regions in chromosome arm basements, normally ␤-heterochromatic, acquire a distinct banding pattern, i.e., become euchromatic by morphological criteria; (iii) an additional bulk of polytenized material arises between the arms of chromosome 3 to form a fragment with a typical banding pattern. Chromosome 2 PH reveals additional ␣-heterochromatin. Su(UR)ES does not affect the viability, fertility, or morphological characters of the imago, and has semidominant expression in the heterozygote and distinct maternal effect. The results obtained provide evidence that the processes leading to DNA underreplication in IH and PH are affected by the same genetic mechanism.

The effects of SUUR protein suggest its role in repressive chromatin renewal during replication inDrosophila

“…Ultimately this results in complete polytenization of these regions in polytene chromosomes. 13 Second, the decrease in H3K27me3 histones observed in SuUR mutants 11 is, in fact, a result of gradual dilution of H3K27me3 mark in repressed chromatin regions (Fig. 2B).…”

“…15 In other aspects SuUR mutants are viable, fertile and show no overt changes in gene expression and Origin Recognition Complex (ORC) binding. 11,13 It is worth mentioning that SuUR is a fast-evolving gene that was found only in Diptera. 16 Despite the low evolutionary conservation, recent studies revealed several important features of Figure 1.…”

“…One of these factors was found in Drosophila. 12,13 In Drosophila melanogaster polytene chromosomes, late-replicating heterochromatic regions often fail to complete replication and remain underreplicated (Fig. 1).…”

“…14 Mutation in the Suppressor of Underreplication (SuUR) gene results in full polytenization of underreplicated regions in chromosome arms and partial polytenization of pericentric regions. 13 This means that under-replication is an active process, which specifically requires functional SUUR protein. SUUR is associated with heterochromatin and acts as a weak position effect modifier.…”

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

“…12 Consequently, these regions undergo full polytenization, as confirmed by direct DNA copy number measurements. 13,14 The SuUR gene encodes for a 962 aa long protein. The N-terminus of the protein (aa 1-250) is moderately homologous to the N-terminal part of an ATPase/helicase domain found in a SNF2/SWI2 family of proteins comprising many well-known chromatin remodelers.…”