DISTRIBUTION OF B52 WITHIN A CHROMOSOMAL LOCUS DEPENDS ON THE LEVEL OF TRANSCRIPTION

Drosophila B52 protein is a homologue of human ASF/SF2 that functions in vitro as an essential pre-mRNA splicing factor. Immunofluorescence analysis of polytene chromosomes has shown that B52 generally colocali zes with RNA polymerase II; however, in contrast to other splicing fac tors, B52 brackets RNA polymerase II at highly active heat-shock puffs . Also, UV cross-linking in nonpolytene cells has shown that B52 cross -links in vivo to DNA flanking the highly active transcription units. Here, we find that the distribution of crosslinked B52 at heat-shock l oci depends on transcription levels. Heat shocks at low and moderate t emperatures, which induce corresponding levels of transcription, recru it B52 both to transcribed DNA and to flanking DNA, whereas a full hea t-shock induction concentrates B52 on the DNA that brackets the entire activated region. We have also identified a 46-kDa protein from Chiro nomus tentans that binds Drosophila B52 antibodies and has a distribut ion on chromosomes analogous to B52. This protein is found throughout the moderately transcribed Balbiani rings. However, when transcription at these rings is hyperinduced to levels comparable to fully induced Drosophila heat-shock genes, the protein is restricted to the boundari es of highly decondensed chromatin. We suggest that B52 tracks to chro matin fibers that are folding or unfolding, and we discuss this in lig ht of B52's proposed roles in pre-mRNA splicing and control.