DNA-interactions and nuclear localisation of the chromosomal HMG domain protein SSRP1 from maize

The structure-specific recognition protein 1 (SSRP1) is a member of the pro tein family containing a high mobility group (HMG) domain DNA-binding motif . We have functionally characterised the 71.4 kDa Zm-SSRP1 protein from mai ze. The chromatin-associated Zm-SSRP1 is detected by immunoblot analysis in maize leaves, kernels and suspension culture cells, but not in roots. Medi ated by its HMG domain, recombinant Zm-SSRP1 interacts structure-specifical ly with supercoiled DNA and DNA minicircles when compared with linear DNA. In linear duplex DNA, the protein does not recognise a specific sequence, b ut it binds preferentially to sequences containing the deformable dinucleot ide TG, as demonstrated by a random oligonucleotide selection experiment. Z m-SSRP1 modulates DNA structure by bending the target sequence, since it pr omotes the circularisation of short DNA fragments in the presence of DNA li gase. Moreover, Zm-SSRP1 facilitates the formation of nucleoprotein structu res, as measured using the bacterial site-specific beta-mediated recombinat ion reaction. Analysis of the subcellular localisation of various SSRP1-GFP fusions revealed that, in contrast to HMG domain transcription factors, th e nuclear localisation sequence of Zm-SSRP1 is situated within a 20-amino a cid residue region adjacent to the HMG domain rather than within the DNA-bi nding domain. The results are discussed in the context of the likely functi on of SSRP1 proteins in transcription and replication.