Initial stages in the Rhizobium-legume symbiosis can be thought of as a reciprocal molecular conversation: transmission of a gene inducer from legume host to bacterium, with ensuing bacterial synthesis of a morphogen that is transmitted to the plant, switching the developmental fate of the legume root. These signal molecules have a key role in determining bacterium-host specificity and the purified Nod factor compounds provide useful new tools to probe plant cell function.

Advances in the understanding of molecular biology of human disease and the development of efficient gene transfer techniques have resulted in practical approaches to human gene therapy, with new techniques being developed at an increasing rate. The first trials have now begun in humans and initial results are positive.

Proteins presented to the immune system must first be cleaved to small peptides by intracellular proteinases. Proteasomes are proteolytic complexes that degrade cytosolic and nuclear proteins. These particles have been implicated in ATP-ubiquitin-dependent proteolysis and in the processing of intracellular antigens for cytolytic immune responses.

The field of capillary electrophoresis has undergone rapid development over the past 10 years. Many advances have been made, but there are still some problems that must be addressed before the technique can reach its full potential.

Programmed cell death occurs in most animal tissues at some stage of their development, but the molecular mechanism by which it is executed is unknown. For some mammalian cells, programmed death seems to occur by default unless suppressed by signals from other cells. Such dependence on specific survival signals provides a simple way to eliminate misplaced cells, for regulating cell numbers and, perhaps, for selecting the fittest cells. But how general is this dependence on survival signals?

The introduction of human insulin to treat diabetics seemed straightforward. What can account for the problems that have followed?

Many of the membrane-associated oxidases that catalyse respiratory reduction of O2 to water simultaneously couple this exergonic reaction to the translocation of protons across the inner mitochondrial membrane, or the cell membrane in prokaryotes, a process by which metabolic energy is conserved for subsequent synthesis of ATP. The molecular mechanism of O2 reduction and its linkage to H+ translocation are now emerging. The bimetallic haem iron-copper reaction centre in this family of enzymes is the critical structure for catalysis of both these processes.

Recent palaeontological discoveries and the correspondence between molecular and morphological results provide fresh insight on the deep structure of mammalian phylogeny. This new wave of research, however, has yet to resolve some important issues.

It is remarkable that the taxonomy and phylogenetic relationships of the earliest known representatives of our own genus, Homo, remain obscure. Advances in techniques for absolute dating and reassessments of the fossils themselves have rendered untenable a simple unilineal model of human evolution, in which Homo habilis succeeded the australopithecines and then evolved via H. erectus into H. sapiens-but no clear alternative consensus has yet emerged.

Sporulation in Bacillus subtilis is a model for how cells of one type generate other differentiated cell types. During sporulation two cellular compartments arise that differ from each other and from the progenitor cell. Differential gene expression between the two is governed by the successive appearance of four transcription factors whose activities are coordinated in crisscross fashion between the two cells.

Called the "mystery of mysteries" by Darwin, speciation is still a little-understood area of evolution. Genetic analysis, however, has yielded new generalizations about speciation and suggests promising avenues of research.

A combination of biochemistry in animal cell-free systems and genetics in yeast is revealing the molecular machinery of the secretory pathway of eukaryotes. Transporting vesicles have a simple coat structure and employ a general mechanism for fusion that is conserved in evolution.

The increasingly detailed biochemical definition of the protein complexes that regulate gene transcription has led to the re-emergence of questions about the role of histones. Much recent evidence suggests that transcriptional activation requires that transcription factors successfully compete with histones for binding to promoters, and that there may be more than one mechanism by which this is achieved.

Sources of organic molecules on the early Earth divide into three categories: delivery by extraterrestrial objects; organic synthesis driven by impact shocks; and organic synthesis by other energy sources (such as ultraviolet light or electrical discharges). Estimates of these sources for plausible end-member oxidation states of the early terrestrial atmosphere suggest that the heavy bombardment before 3.5 Gyr ago either produced or delivered quantities of organics comparable to those produced by other energy sources. Which sources of prebiotic organics were quantitatively dominant depends strongly on the composition of the early terrestrial atmosphere. In the event of an early strongly reducing atmosphere, production by atmospheric shocks seems to have dominated that due to electrical discharges. Organic synthesis by ultraviolet light may, in turn, have dominated shock production, but only if a long-wavelength absorber such as H2S were supplied to the atmosphere at a rate sufficient for synthesis to have been limited by ultraviolet flux, rather than by reactant abundance. In the apparently more likely case of an early terrestrial atmosphere of intermediate oxidation state, atmospheric shocks were probably of little importance for direct organic production. For [H2]/[CO2] ratios of approximately 0.1, net organic production was some three orders of magnitude lower than for reducing atmospheres, with delivery of intact exogenous organics in interplanetary dust particles (IDPs) and ultraviolet production being the most important sources. At still lower [H2]/[CO2] ratios, IDPs may have been the dominant source of prebiotic organics on the early Earth. Endogenous, exogenous and impact-shock sources of organics could each have made a significant contribution to the origins of life.

In the cell, as in vitro, the final conformation of a protein is determined by its amino-acid sequence. But whereas some isolated proteins can be denatured and refolded in vitro in the absence of other macromolecular cellular components, folding and assembly of polypeptides in vivo involves other proteins, many of which belong to families that have been highly conserved during evolution.

The structures of several classes of DNA-binding domains reveal a variety of designs for recognizing a specific site on DNA.

There is now very persuasive evidence that the transmissible agent for spongiform encephalopathies such as scrapie, consists of a modified form of the normal host protein PrPc, devoid of any nucleic acid. On the other hand, because there are many different strains of scrapie agent with distinct phenotypes which can be propagated in animals homozygous for the PrPc gene, it has been suggested that a nucleic acid must be a component of the agent. Can the two views be reconciled?

Mars is believed to be lifeless, but it may be possible to transform it into a planet suitable for habitation by plants, and conceivably humans. The success of such an enterprise would depend on the abundance, distribution and form of materials on the planet that could provide carbon dioxide, water and nitrogen.