Accepted_test

Genome expression can be regulated by the speed of transcription. The rate of nascent RNA synthesis is unusually high in lampbrush chromosomes. This is most likely achieved by the tight coverage of transcribed genes by elongating RNA polymerases. Recently, we established that at the lampbrush chromosome stage, avian oocytes accumulate enormous amounts of mRNAs, long non-coding RNAs and small non-coding RNAs that are synthesised in the oocyte nucleus (https://doi.org/10.1101/2024.02.05.577752). We concluded that in avian oocytes, hypertranscription on the lateral loops of giant lampbrush chromosomes is a primary mechanism to synthesise large amounts of maternal RNAs for each of the transcribed genes. Since we detected unspliced or partially spliced nuclear transcripts for many genes, we aimed to detect nascent transcripts and characterize the organision of transcription units in situ. By RNA-FISH we visualised transcription units of a number of protein-coding genes on chicken lampbrush chromosomes. For all studied examples, we observed complete correspondence of transcript detection by nuclear RNA-seq and by RNA-FISH with nascent transcripts. We argue that nuclear RNA-seq profile allows predicting the chromomere-loop pattern of lampbrush chromatin domains. We conclude that lampbrush chromosomes provide an especially useful system for studying thousands of transcription loops as well as the packaging of transcriptionally active chromatin and nascent RNAs.