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.

The biliprotein phytochrome regulates plant growth and developmental responses to the ambient light environment through an unknown mechanism. Biochemical analyses demonstrate that phytochrome is an ancient molecule that evolved from a more compact light sensor in cyanobacteria. The cyanobacterial phytochrome Cph1 is a light-regulated histidine kinase that mediates red, far-red reversible phosphorylation of a small response regulator, Rcp1 (response regulator for cyanobacterial phytochrome), encoded by the adjacent gene, thus implicating protein phosphorylation-dephosphorylation in the initial step of light signal transduction by phytochrome.

Plants constantly monitor their light environment in order to grow and develop optimally, in part through use of the phytochromes, which sense red/far-red light. A phytochrome binding protein, PKS1 (phytochrome kinase substrate 1), was identified that is a substrate for light-regulated phytochrome kinase activity in vitro. In vivo experiments suggest that PKS1 is phosphorylated in a phytochrome-dependent manner and negatively regulates phytochrome signaling. The data suggest that phytochromes signal by serine-threonine phosphorylation.

The discovery of cyanobacterial phytochrome histidine kinases, together with the evidence that phytochromes from higher plants display protein kinase activity, bind ATP analogs, and possess C-terminal domains similar to bacterial histidine kinases, has fueled the controversial hypothesis that the eukaryotic phytochrome family of photoreceptors are light-regulated enzymes. Here we demonstrate that purified recombinant phytochromes from a higher plant and a green alga exhibit serine͞threonine kinase activity similar to that of phytochrome isolated from dark grown seedlings. Phosphorylation of recombinant oat phytochrome is a light-and chromophore-regulated intramolecular process. Based on comparative protein sequence alignments and biochemical cross-talk experiments with the response regulator substrate of the cyanobacterial phytochrome Cph1, we propose that eukaryotic phytochromes are histidine kinase paralogs with serine͞threonine specificity whose enzymatic activity diverged from that of a prokaryotic ancestor after duplication of the transmitter module.