MRN1 - encoding a novel mRNA binding protein - is a multicopy suppressor of RSC - Nhp6 synthetic lethality.
Louis Düring, Steen Holmberg
Department of Genetics, University of Copenhagen, Oesterfarimagsgade2A, Copenhagen, DK-1353, Denmark
The S. cerevisiae architectural factors, the n on- h istone proteins Nhp6A and Nhp6B belong to the HMGB family of HMG proteins. The HMGB proteins are abundant, evolutionary conserved and have a general function in the cell as structural components of chromatin to achieve a proper promoter architecture. The RSC complex is a 16 subunit complex conserved from yeast to metazoans with central roles in gene regulation. Nhp6p interacts genetically with RSC and SWI/SNF. We selected multicopy suppressors of the growth defect of a Δ nhp6A Δ nhp6B rsc8-ts16 strain and isolated YPL184c, which we denote MRN1 for m ulticopy suppressor of R SC - N hp6 synthetic lethality. Mrn1p contains a RNA processing and modification motif and two RNA recognition motifs. The only homologue with known biological function is S. pombe Nrd1p, a negative regulator of the onset of sexual differentiation. Employing chromatin immunoprecipitation we show that Mrn1p is present both at the promoter and at the entire coding region of Nhp6A/B and RSC regulated genes. Co-immunoprecipitation demonstrates that Mrn1p interacts with the RNAPII subunit Rpb3p in vivo. Interestingly, a modified chromatin immunoprecipitation procedure, RNA immunoprecipitation, shows that Mrn1p is present at actively transcribed genes and that it associates with the nascent RNA emanating from elongating RNAPII. These results support a role for Mrn1p in the interface between chromatin, transcriptional initiation and mRNA processing.