Identification and characterization of Polycomb group genes in the silkworm, Bombyx mori Zhiqing Li • Tsuneyuki Tatsuke • Kosuke Sakashita • Li Zhu • Jian Xu • Hiroaki Mon • Jae Man Lee • Takahiro Kusakabe Received: 24 July 2011 / Accepted: 12 December 2011 / Published online: 21 December 2011 Ó Springer Science+Business Media B.V. 2011 Abstract Polycomb group (PcG) proteins are involved in chromatin modifications for maintaining gene repression that play important roles in the regulation of gene expres- sion, tumorigenesis, chromosome X-inactivation, and genomic imprinting in Drosophila melanogaster, mam- mals, and even plants. To characterize the orthologs of PcG genes in the silkworm, Bombyx mori, 13 candidates were identified from the updated silkworm genome sequence by using the fruit fly PcG genes as queries. Comparison of the silkworm PcG proteins with those from other insect species revealed that the insect PcG proteins shared high sequence similarity. High-level expressions of all the silkworm PcG genes were maintained through day 2 to day 7 of embryogenesis, and tissue microarray data on day 3 of the fifth instar larvae showed that their expression levels were relatively low in somatic tissues, except for Enhancer of zeste (E(Z)). In addition, knockdown of each PRC2 com- ponent, such as E(Z), Extra sex combs (ESC), and Sup- pressor of zeste 12 (SU(Z)12), considerably decreased the global levels of H3K27me3 but not of H3K27me2. Taken together, these results suggest that insect PcG proteins are highly conserved during evolution and might play similar roles in embryogenesis. Keywords Bombyx mori Á Polycomb group genes Á Comparison analysis Á Expression patterns Á RNA interference Introduction Polycomb group (PcG) proteins were originally identified as repressors to regulate Hox gene expression in fruit fly (D. melanogaster)[1, 2], and are then found to distribute widely in many other species, including mammals [3] and plants [4]. To date, PcG proteins have been implicated in various biological processes, such as the cell cycle pro- gram, tumorigenesis, chromosome X-inactivation, genomic imprinting, and so on [5–8]. In general, there are three kinds of mutually cooperative PcG complexes: Polycomb repressive complex 1 (PRC1), Polycomb repressive complex 2 (PRC2), and Pleiohomeotic repressive complex (PhoRC) [9–11]. The proposed model for a transcriptional repression mechanism by PcG proteins is as follows: the DNA binding protein presented in Pho-RC recognizes Polycomb responsive elements (PREs) in the target genes and then recruits the PRC2 complex to these loci. The PRC2 complex catalyzes the tri-methylation at lysine 27 of histone 3 (H3K27me3). Subsequently, the PRC1 complex recognizes H3K27me3 and mediates transcriptional repres- sion [3, 12]. The detailed repression mechanism might be more complicated and varied among different species. Recently, genome-wide identification of PcG target genes in fruit fly and mammals have provided the global profiles of PcG binding sites [13–15]. However, how PcG proteins are recruited to these targets remains largely unknown. Fruit fly PhoRC core components include Pleiohomeotic (Pho) and SCM-related gene containing four MBT domains (Sfmbt) [11, 16]. It is reported that Pho is the only Electronic supplementary material The online version of this article (doi:10.1007/s11033-011-1362-5) contains supplementary material, which is available to authorized users. Z. Li Á T. Tatsuke Á K. Sakashita Á L. Zhu Á J. Xu Á H. Mon Á J. M. Lee Á T. Kusakabe (&) Laboratory of Silkworm Science, Kyushu University Graduate School of Bioresource and Bioenvironmental Sciences, Hakozaki 6-10-1, Fukuoka 812-8581, Japan e-mail: kusakabe@agr.kyushu-u.ac.jp 123 Mol Biol Rep (2012) 39:5575–5588 DOI 10.1007/s11033-011-1362-5