The Drosophila SNR1 (SNF5/INI1) Subunit Directs Essential Developmental Functions of the Brahma Chromatin Remodeling Complex

The coordination between cell proliferation and terminal differentiation is of critical importance during the development of multicellular organisms. The metazoan SWI/SNF-related protein complexes function in both of these processes in a broad range of tissues by altering gene expression at specific promoters through ATP-dependent chromatin remodeling (32). The Saccharomyces cerevisiae SWI/SNF complex is a ∼2-MDa multisubunit assembly capable of ATP-dependent chromatin remodeling both in vitro and in vivo (53). Highly related complexes have also been identified in flies (23, 48) and mammals (73). Among the SWI/SNF complex proteins, none appears to be capable of sequence-specific DNA binding; thus, the precise mechanism for targeting the SWI/SNF complexes to their respective gene targets in vivo is not well understood. The complex can interact with transcription factors to facilitate SWI/SNF function at specific promoters (53). Interactions have been observed with steroid hormone receptors; EKLF (erythroid Kruppel-like factor), which activates the β-globin locus; C/EBPβ, which activates transcription of myeloid genes (35); c-Myc (13, 65); and HRX (56), a homolog of Drosophila melanogaster Trithorax (TRX) that regulates expression of the homeotic genes. Depending on the chromatin context of the binding site, some transcription factors may initially bind to their recognition site and then recruit SWI/SNF to alleviate chromatin repression at the promoter, or conversely, the factor may require SWI/SNF activity for binding. While there is presently very little known about what types of protein interactions might promote such recruitment, several complex subunits may function coordinately to mediate these associations (46). Further, mounting evidence suggests that the various SWI/SNF and related complexes might function in both gene activation and repression (22, 64). Studies of the fly Brm and mammalian hBrm/Brg1 complexes have revealed widespread and important biological roles in development (45, 66). In addition to transcriptional activation, the complex functions in regulating cell cycle progression through interactions with the retinoblastoma (Rb) protein and/or cyclin-cdk complexes (7). The purified Brm complex contains at least eight proteins, including BRM, MOR, SNR1, and OSA (15, 48). While brm, mor, and osa were identified in genetic screens as dosage-dependent activators of homeotic gene transcription, counteracting the repressive effects of the Polycomb group of genes (31), none of the other Brm complex genes was identified in any genetic screen. The snr1 gene (SNF5-Related-1) was identified (23) by molecular means based on its similarity to the human INI1/SMARCB1/hSNF5 gene, which encodes a core subunit of the hBrm and Brg1 complexes. Both snr1 and INI1 are strongly conserved with SNF5, which encodes an essential component of the yeast SWI/SNF complex, and SNR1, INI1, and SNF5 are required for full function of their respective complexes in vivo and in vitro (23, 38, 39, 55). While brm, mor, and osa have dosage-sensitive mutant phenotypes in combination with other gene mutations that can be examined in the imaginal tissues, such as leg and wing disks (8, 9, 24, 67), the existing snr1 null mutations do not display significant dosage sensitivity in most genetic assays, apparently inconsistent with SNR1 serving as a core subunit of the Brm complex and making analysis of this essential gene very difficult. Thus, the snr1 gene product is either in excess of that required to assemble the Brm complex, has unidentified functions independent of the complex, or has a limited role at a subset of target genes and thus is dispensable for some Brm complex functions. To help distinguish among these possibilities, we carried out a large-scale noncomplementation chemical mutagenesis screen to identify conditional mutant alleles of snr1 (77), and we report here the molecular and genetic characterization of snr1E1, which exhibits recessive lethality at 29°C and a variety of temperature-dependent phenotypes. Our genetic and biochemical characterizations of snr1E1 reveal that snr1 has essential functions in the development of many tissues including the peripheral nervous system, eye, and wing veins, and surprisingly, it is also required for sustained adult viability. Further, our analyses provide strong links among snr1 function, chromatin remodeling, and the regulation of cell proliferation, confirming its predicted role in restricting cell growth during normal development.