The scarcity of usable nitrogen frequently limits plant growth. A tight metabolic association with rhizobial bacteria allows legumes to obtain nitrogen compounds by bacterial reduction of dinitrogen (N2) to ammonium (NH4+). We present here the annotated DNA sequence of the alpha-proteobacterium Sinorhizobium meliloti, the symbiont of alfalfa. The tripartite 6.7-megabase (Mb) genome comprises a 3.65-Mb chromosome, and 1.35-Mb pSymA and 1.68-Mb pSymB megaplasmids. Genome sequence analysis indicates that all three elements contribute, in varying degrees, to symbiosis and reveals how this genome may have emerged during evolution. The genome sequence will be useful in understanding the dynamics of interkingdom associations and of life in soil environments.

We identified four proteins in nuclear extracts from HeLa cells which specifically bind to a scaffold attachment region (SAR) element from the human genome. Of these four proteins, SAF‐A (scaffold attachment factor A), shows the highest affinity for several homologous and heterologous SAR elements from vertebrate cells. SAF‐A is an abundant nuclear protein and a constituent of the nuclear matrix and scaffold. The homogeneously purified protein is a novel double stranded DNA binding protein with an apparent molecular weight of 120 kDa. SAF‐A binds at multiple sites to the human SAR element; competition studies with synthetic polynucleotides indicate that these sites most probably reside in the multitude of A/T‐stretches which are distributed throughout this element. In addition we show by electron microscopy that the protein forms large aggregates and mediates the formation of looped DNA structures.

'Corresponding author Communicated by R.Knippers p53 has been reported to inhibit the DNA helicase intrinsic to simian virus 40 large tumor antigen (T antigen). We found that inhibition is not restricted to T antigen, but also affects several other DNA and RNA helicases. Complexing of the helicases by the p53 protein as a possible inactivation mechanism could be excluded. Instead, the anti-helicase activity can be explained by our finding that p53 binds with high affinity to singlestranded nucleic acids and has a strong DNA-DNA and RNA-RNA annealing activity. We could also show that p53 is able to alter the secondary structure of RNA and/or to influence dynamic RNA-RNA interactions. These results, and the fact that the affinity of p53 to RNA is about one order of magnitude higher than to singlestranded DNA, imply an RNA-specific function of p53 in vivo.

The higher plant Arabidopsis thaliana (Arabidopsis) is an important model for identifying plant genes and determining their function. To assist biological investigations and to define chromosome structure, a coordinated effort to sequence the Arabidopsis genome was initiated in late 1996. Here we report one of the first milestones of this project, the sequence of chromosome 4. Analysis of 17.38 megabases of unique sequence, representing about 17% of the genome, reveals 3,744 protein coding genes, 81 transfer RNAs and numerous repeat elements. Heterochromatic regions surrounding the putative centromere, which has not yet been completely sequenced, are characterized by an increased frequency of a variety of repeats, new repeats, reduced recombination, lowered gene density and lowered gene expression. Roughly 60% of the predicted protein-coding genes have been functionally characterized on the basis of their homology to known genes. Many genes encode predicted proteins that are homologous to human and Caenorhabditis elegans proteins.

We show that SAF-A, a nuclear protein which specifically binds vertebrate scaffold-attachmentregion (SAR) elements with high affinity is identical with hnRNP-U, assumed to be involved in packaging of hnRNA in ribonucleoprotein particles. Ultraviolet cross-linking experiments show that the protein, referred to as hnRNP-U/SAF-A, is bound to chromosomal DNA in vivo. Zn vitro, the isolated protein binds to double-stranded and single-stranded DNA and forms higher ordered nucleic-acid -protein complexes. Filter-binding experiments performed with different types of natural and synthetic nucleic acids as substrates show that the protein binds DNA and RNA with different affinities and most likely at different binding sites. We conclude that hnRNP-U/SAF-A thus may have functions in the organisation of chromosomal DNA in addition to its suggested role in hnRNA metabolism.It is widely believed that DNA in eukaryotic interphase nuclei is organized into loop domains with average lengths of 50-100 kb and that these chromatin loops are bound to internal nuclear structures via the scaffold-attachment region (SAR; for review see [l]) or the matrix-associated region (MAR; for review see 12, 31) DNA elements. Many DNA fragments which specifically bind to internal nuclear structures have been identified, but only little is known about their function in vivo and how they exert this function. In addition, only a few proteins ([4, 51 and references therein) are known which possibly are involved in the binding of these DNA fragments to internal nuclear structures.We have therefore attempted to analyse for nuclear proteins which specifically bind SAR DNA elements and we reported recently on the purification of a DNA binding protein with this property 161. This protein, SAF-A (scaffoldattachment factor A), is a component of the nuclear matrix and of scaffolds; it specifically binds vertebrate SAR DNA elements and reconstitutes looped structures with these DNAs [6]. SAF-A is an abundant protein and the large number of molecules presenthucleus indicate a structural function of this protein. In this communication, we identify SAF-A as hnRNP-U [7], a protein component of heterogeneous nuclear ribonucleoprotein complexes (for review see [S-lo]). We refer to this protein as hnRNP-U/SAF-A and characterize the nucleic-acid binding properties of the protein. We further show by ultraviolet (UV) cross-linking experiments that the protein is bound to DNA in vivo and disCorrespondence to A. Richter,