Genomic imprinting,representing parent-specific expression of alleles at a locus,is mainly evident in flowering plants and placental mammals.Most imprinted genes,including numerous non-coding RNAs,are located in clusters regulated by imprinting control regions(ICRs).The acquisition and evolution of genomic imprinting is among the most fundamental genetic questions.Discoveries about the transition of mammalian imprinted gene domains from their non-imprinted ancestors,especially recent studies undertaken on the most ancient mammalian clades-the marsupials and monotremes from which model species genomes have recently been sequenced,are of high value.By reviewing and analyzing these studies,a close connection between non-coding RNAs and the acquisition of genomic imprinting in mammals is demonstrated.The evidence comes from two observations accompanied with the acquisition of the imprinting:(i) many novel non-coding RNA genes emerged in imprinted regions;(ii) the expressions of some conserved non-coding RNAs have changed dramatically.Furthermore,a systematical analysis of imprinted snoRNA(small nucleolar RNA) genes from 15 vertebrates suggests that the origination of imprinted snoRNAs occurred after the divergence between eutherians and marsupials,followed by a rapid expansion leading to the fixation of major gene families in the eutherian ancestor prior to the radiation of modern placental mammals.Involved in the regulation of imprinted silencing and mediating the chromatins epigenetic modification may be the major roles that non-coding RNAs play during the acquisition of genomic imprinting in mammals.
ZHANG YiJun & QU LiangHu Key Laboratory of Gene Engineering of the Ministry of Education,State Key Laboratory for Biocontrol,Sun Yan-Sen University,Guangzhou 510275,China
From a specialized cDNA library, four novel box H/ACA snoRNAs, named GLsR22, GLsR23, GLsR24 and GLsR25, were identified from the primitive eukaryote, Giardia lamblia. Bioinformat- ics analyses indicated that all of them can be poten- tially folded into double hairpins, the typical secon- dary structures of box H/ACA snoRNAs. GLsR24 and GLsR25 are predicted to guide the site-specific pseudouridylation at U1753 and U2396 on 23S rRNA, respectively, while GLsR22 and GLsR23 belong to the family of “orphan” snoRNAs. All of the four novel snoRNAs are encoded by single copy genes and located in small intergenic regions. Interestingly, compared with the counterparts previously reported in Archaea and other unicellular protozoan, the box H/ACA snoRNAs identified from G. lamblia have unique structural features, implying that snoRNAs evolved from prokaryotes to eukaryotes in different ways.
LUO Jun ZHOU Hui CHEN Chongjian LI Yan CHEN Yueqin QU Lianghu
