PURIFICATION, CLONING, AND PRELIMINARY CHARACTERIZATION OF A SPIROPLASMA-CITRI RIBOSOMAL-PROTEIN WITH DNA-BINDING CAPACITY

The rpsB-tsf-x operon of Spiroplasma citri encodes ribosomal protein S 2 and elongation factor Ts, two components of the translational appara tus, and an unidentified X protein. A potential DNA-binding site (a 20 -base pair (bp) inverted repeat sequence) is located at the 3' end of rpsB. Southwestern analysis of S. citri proteins, with a 30-bp double- stranded oligonucleotide probe (IRS), containing the 20-bp inverted re peat sequence and the genomic flanking sequences, detected an IRS-bind ing protein of 46 kDa (P46). P46 protein, which displays preferential affinity for the IRS, was purified from S. citri by a combination of a ffinity and gel filtration chromatographies. The native form of P46 se ems to be homomultimeric as estimated by SDS-polyacrylamide gel electr ophoresis analysis and gel filtration. A 3.5-kilobase pair S. citri DN A fragment comprising the P46 gene and flanking sequences was cloned a nd sequenced. Sequence analysis of this DNA fragment indicated that th e P46 gene is located within the S10-spc operon of S. citri at the pos ition of the gene coding for ribosomal protein L29 in the known S10-sp c operons, The similarity between the N-terminal domain of P46 and the L29 ribosomal protein family and the presence of a 46-kDa IRS-binding protein in S. citri ribosomes indicated that P46 is the L29 ribosomal protein of S. citri, We suggest that P46 is a bifunctional protein wi th an L29 N-terminal domain and a C-terminal domain involved in IRS bi nding.