THE DROSOPHILA SUPPRESSOR OF SABLE PROTEIN BINDS TO RNA AND ASSOCIATES WITH A SUBSET OF POLYTENE CHROMOSOME BANDS

Mutations of the Drosophila melanogaster suppressor of sable [su(s)] g ene, which encodes a 150-kDa nuclear protein [Su(s)], increase the acc umulation of specific transcripts in a manner that is not well underst ood but that appears to involve pre-mRNA processing, Here, we report b iochemical analysis of purified, recombinant Su(s) [rSu(s)] expressed in baculovirus and in Escherichia coli as maltose binding protein (MBP ) fusions and immunocytochemical analysis of endogenous Su(s), This wo rk has shown that purified, baculovirus-expressed rSu(s) binds to RNA in vitro with a high affinity and limited specificity, Systematic evol ution of ligands by exponential enrichment was used to identify prefer red RNA targets of rSu(s), and a large proportion of RNAs isolated con tain a full or partial match to the consensus sequence UCAGUAGUCU, whi ch was confirmed to be a high-affinity rSu(s) binding site, An MBP-Su( s) fusion protein containing the N-terminal third of Su(s) binds RNAs containing this sequence with a higher specificity than full-length, b aculovirus-expressed rSu(s), The consensus sequence resembles both a c ryptic 5' splice site and a sequence that is found near the 5' end of some Drosophila transcripts, Immunolocalization studies showed that en dogenous Su(s) is distributed in a reticulated pattern in Drosophila e mbryo and salivary gland nuclei, In salivary gland cells, Su(s) is fou nd both in the nucleoplasm and in association with a subset of polyten e chromosome bands. Considering these and previous results, we propose two models to explain how su(s) mutations affect nuclear pre-mRNA pro cessing.