Ribosome components are associated with sites of transcription

It is generally believed that eukaryotic ribosomes first associate with mRNA in the cytoplasm. However, we show with chromosomal immunostaining and in situ hybridization that ribosomal subunits are present at transcription sites of Drosophila salivary gland chromosomes. Immunostaining was carried out with antibodies specific for 27 ribosomal proteins, two translation factors and one that specifically recognizes rRNA. In situ hybridization was with several probes specific for both rRNA subunits. The kinetics of recruitment following transcription initiation suggest that the association is with newly transcribed pol II transcripts. These data indicate that ribosome components associate with nascent RNP complexes within the nucleus.