Non‐coding RNAs are multi‐step genetic expression regulators. Bacterial 6S RNA and murine B2 RNA are non‐coding RNAs able to bind and inhibit RNA polymerase II (RNAPII). Seeking for non‐coding RNAs bound to human RNAPII, immunoprecipitations against RNAPII were carried on mitotic HeLa cells. Co‐immunoprecipitated RNAs were purified and labelled and U1 RNA was found to be peculiarly enriched versus the control IP. This coimmunoprecipitation reflects the association of the U1 snRNP with RNAPII. In order to verify this associtation on an active transcription site, stable cell lines were established with multiple copies insertion of a gene at one single site, allowing its visualisation by FISH (Fluorescent In Situ Hybridization). Two distincts cell lines were made with two similars contructs only differing by three point mutations abolishing the splicing in one construct. Whereas 4 snRNAs were absent on a non splicing transcription site, U1 snRNA is recruited both on spliced and non‐spliced transcription sites. The whole U1snRNP seems to be recruited independently of splicing because of the presence of U1 specific proteins. These results may suggests a role for the RNAPII - U1 snRNP interaction in cotranscriptionnal splicing.
