RNA- and single-stranded DNA-binding (SSB) proteins expressed during Drosophila melanogaster oogenesis: a homolog of bacterial and eukaryotic mitochondrial SSBs.

Little is known about the identity and involvement of single-stranded (ss) DNA-binding (SSB) and RNA-binding proteins in developmental processes 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 directional ovarian cDNA library and conducted expression cloning screens which identified five unique cDNAs that encode proteins capable of binding ssDNA. All five represent previously unreported sequences. The remainder of this paper focuses on one of these cDNAs which encodes a Dm protein displaying significant sequence homology to Escherichia coli ssDNA-binding protein (SSB, involved in DNA replication, repair and recombination), as well as eukaryotic SSBs isolated from the mitochondria (mt) of rats, frogs, humans and yeast. The deduced amino acid (aa) sequence of this 15.6-kDa protein, which we will refer to as Dm mtSSB, displays average identities of 38.3% with eukaryotic mtSSBs and 23.4% with 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 expression appears to be developmentally regulated. It is expressed as a single 600-nucleotide (nt) transcript during oogenesis and embryogenesis. A larger transcript of 1500 nt is prevalent in some later stages of Dm development.