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The relationship between stochastic transcriptional bursts and dynamic 3D chromatin states is not well understood. Using an innovated, ultra-sensitive technique, we address here enigmatic features underlying the communications between MYC and its enhancers in relation to the transcriptional process. MYC thus interacts with its flanking enhancers in a mutually exclusive manner documenting that enhancer hubs impinging on MYC detected in large cell populations likely do not exist in single cells. Dynamic encounters with pathologically activated enhancers responsive to a range of environmental cues, involved <10% of active MYC alleles at any given time in colon cancer cells. Being the most central node of the chromatin network, MYC itself likely drives its communications with flanking enhancers, rather than vice versa. We submit that these features underlie an acquired ability of MYC to become dynamically activated in response to a diverse range of environmental cues encountered by the cell during the neoplastic process., (© The Author(s) 2020. Published by Oxford University Press on behalf of Nucleic Acids Research.)

Irradiation with ultraviolet light was used to induce covalent linkage between hnRNA and its associated proteins in intact HeLa cells, late after infection with adenovirus type 2. Covalently linked hnRNA-protein complexes, containing polyadenylated adenoviral RNA, were isolated and their protein moiety characterized. Host 42,000 Mr hnRNP proteins proved to be the major proteins crosslinked to viral hnRNA. To investigate their possible involvement in RNA processing, the localization of these cross-linked polypeptides on adenoviral late transcripts was determined. Sequences of RNA around the attachment sites of the protein were isolated. After in vitro labeling they were hybridized to Southern blots of adeno DNA fragments. The hybridization patterns revealed that the 42,000 Mr polypeptides can be linked to adenoviral transcripts over the entire length of the RNA, corresponding to 16.2-91.5 m.u. of the viral genome. Fine mapping within the Hind III B region (16.8-31.5 m.u.) established, however, that the localization of the cross-linked polypeptides was not random in all parts of the transcript. Sequences around the third leader and the 3' part of the i-leader were overrepresented, whereas the regions encoding VA I and VA II RNA and the late region 1 mRNA bodies were underrepresented in the cross-linked RNA. Using genomic DNA fragments and a cDNA clone containing the tripartite leader it appeared that leader and intervening sequences were represented about equally in cross-linked RNA fragments. Although these results do not support the notion that introns or exons are specifically interacting with one RNP protein, they demonstrate that the 42,000 hnRNP proteins are non randomly positioned on the RNA sequence.

Heterogeneous nuclear protein complexes (hnRNP) containing the precursor RNA from the adenovirus early region 2 were analysed to determine the specificity of protein-RNA interaction. RNA precursor sequences were present in isolated hnRNP complexes and endogenous 30S particles. At least 20-40 bases long fragments were protected when RNase A was used to remove unprotected RNA sequences in hnRNA complexes. Similarly around 40 bases of RNA were protected in 30S particles. These sequences represent discrete regions of the adenovirus genome. Especially sequences complementary to the EcoRI-F fragment encoding the first leader and the major intron for the DNA binding protein (DBP) RNA precursor, were analysed in detail. Tentatively, sequences resistant to RNase A were located in the middle of the intron and at the splice-donor junction of the first leader of the DBP precursor RNA. The same sequences were identified irrespective whether hnRNP complexes or 30S particles were used suggesting that 30S particles originate from hnRNP complexes. A 38.000 dalton protein appears to be in direct contact with RNA sequences complementary to the EcoRI-F fragment.

Ultraviolet light induced RNA-protein cross-linking for identification of polypeptides interacting with RNA in intact cells (Wagenmakers et al. 1980), is limited by the intensity of the label in the proteins or in residual nucleotides remaining attached to the proteins after RNase treatment of the RNA-protein complexes. Here we report a method, where th cross-linked RNA-protein complexes are treated with RNase T1 and the T1-oligonucleotides covalently linked to the proteins are labeled in the 5' terminus using gamma-32P-ATP and T4 polynucleotide kinase. The cross-linked proteins can then readily be identified owing to the incorporated 32P label. As examples, proteins associated with polyadenylated mRNA, hnRNA and adenoviral VA RNA were identified. A protein with a molecular weight of approximately 50,000 is found associated with adenovirus-coded VA RNA. This was confirmed by binding assays, in which labeled VAI RNA is incubated with proteins from uninfected and adenovirus infected HeLa cells immobilized on nitrocellulose sheets.