title: The ribosomal RNA transcription unit of Entamoeba invadens: accumulation of unprocessed pre-rRNA and a long non coding RNA during encystation.
abstract: The ribosomal RNA genes in Entamoeba spp. are located on extrachromosomal circular molecules. Unlike model organisms where rRNA transcription stops during growth stress, Entamoeba histolytica continues transcription; but unprocessed pre-rRNA accumulates during stress, along with a novel class of circular transcripts from the 5'-external transcribed spacer (ETS). To determine the fate of rRNA transcription during stage conversion between trophozoite to cyst we analyzed Entamoeba invadens, a model system for differentiation studies in Entamoeba. We characterized the complete rDNA transcription unit by mapping the ends of pre-rRNA and mature rRNAs. The 3' end of mature 28S rRNA was located 321 nt downstream of the end predicted by sequence homology with E. histolytica. The major processing sites were mapped in external and internal transcribed spacers. The promoter located within 146 nt upstream of 5' ETS was used to transcribe the pre-rRNA. On the other hand, a second promoter located at the 3' end of 28S rDNA was used to transcribe almost the entire intergenic spacer into a long non coding (nc) RNA (>10 kb). Interestingly we found that the levels of pre-rRNA and long ncRNA, measured by northern hybridization, decreased initially in cells shifted to encystation medium, after which they began to increase and reached high levels by 72 h when mature cysts were formed. Unlike E. histolytica, no circular transcripts were found in E. invadens. E. histolytica and E. invadens express fundamentally different ncRNAs from the rDNA locus, which may reflect their adaptation to different hosts (human and reptiles, respectively). This is the first description of rDNA organization and transcription in E. invadens, and provides the framework for further studies on regulation of rRNA synthesis during cyst formation.
authors.affiliation: School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, India. Electronic address: sandy.mbt@gmail.com.
authors.forename: Sandeep
authors.initials: S
authors.lastname: Ojha
authors.affiliation: School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, India. Electronic address: nishant900@gmail.com.
authors.forename: Nishant
authors.initials: N
authors.lastname: Singh
authors.affiliation: School of Life Sciences, Jawaharlal Nehru University, New Delhi 110067, India. Electronic address: alok.bhattacharya@gmail.com.
authors.forename: Alok
authors.initials: A
authors.lastname: Bhattacharya
authors.affiliation: School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, India. Electronic address: sbjnu110@gmail.com.
authors.forename: Sudha
authors.initials: S
authors.lastname: Bhattacharya
mesh_terms: D002874:Chromosome Mapping; D021903:DNA, Ribosomal Spacer; D004747:Entamoeba / Q000235:genetics* / Q000254:growth & development; D023061:Gene Order; D011401:Promoter Regions, Genetic; D012322:RNA Precursors / Q000235:genetics*; D062085:RNA, Long Noncoding / Q000235:genetics*; D012335:RNA, Ribosomal / Q000235:genetics*; D014158:Transcription, Genetic
chemical_list: D021903:DNA, Ribosomal Spacer; D012322:RNA Precursors; D062085:RNA, Long Noncoding; D012335:RNA, Ribosomal
keywords: Circular rDNA; Entamoeba invadens; Intergenic spacer transcript; Long non coding RNA; Pre-rRNA accumulation