RNA-STRANDED AND SINGLE-STRANDED DNA-BINDING (SSB) PROTEINS EXPRESSEDDURING DROSOPHILA-MELANOGASTER OOGENESIS - A HOMOLOG OF BACTERIAL ANDEUKARYOTIC MITOCHONDRIAL SSBS

Little is known about the identity and involvement of single-stranded (ss) DNA-binding (SSB) and RNA-binding proteins in developmental proce sses that occur during oogenesis in Drosophila melanogaster (Dm). Here , we describe a molecular approach designed to identify such proteins by virtue of their ssDNA-binding activity. We have constructed a direc tional ovarian cDNA library and conducted expression cloning screens w hich identified five unique cDNAs that encode proteins capable of bind ing ssDNA. All five represent previously unreported sequences. The rem ainder of this paper focuses on one of these cDNAs which encodes a Dm protein displaying significant sequence homology to Escherichia coli s sDNA-binding protein (SSB, involved in DNA replication, repair and rec ombination), as well as eukaryotic SSBs isolated from the mitochondria (mt) of rats, frogs, humans and yeast. The deduced amino acid (aa) se quence of this 15.6-kDa protein, which we will refer to as Dm mtSSB, d isplays average identities of 38.3% with eukaryotic mtSSBs and 23.4% w ith bacterial SSBs. Gel retardation analysis with an affinity-purified GST fusion protein confirms that Dm mtSSB specifically binds ss, but not double stranded DNA. Dm mtSSB is encoded by a nuclear gene whose e xpression appears to be developmentally regulated. It is expressed as a single 600-nucleotide (nt) transcript during oogenesis and embryogen esis. A larger transcript of 1500 nt is prevalent in some later stages of Dm development.