The multicellular green alga Volvox carteri and its morphologically diverse close relatives (the volvocine algae) are well suited for the investigation of the evolution of multicellularity and development. We sequenced the 138–mega–base pair genome of V. carteri and compared its ~14,500 predicted proteins to those of its unicellular relative Chlamydomonas reinhardtii. Despite fundamental differences in organismal complexity and life history, the two species have similar protein-coding potentials and few species-specific protein-coding gene predictions. Volvox is enriched in volvocine-algal–specific proteins, including those associated with an expanded and highly compartmentalized extracellular matrix. Our analysis shows that increases in organismal complexity can be associated with modifications of lineage-specific proteins rather than large-scale invention of protein-coding capacity.

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The soil bacterium Rhizobium meliloti responds to chemotactic stimuli by modulating the rotary speed of its flagella. Unlike in Escherichia coli, the signal transduction chain of R. meliloti contains two different response regulators, CheY1 and CheY2, but no CheZ phosphatase. Phosphorylation of CheY1 and CheY2 by the central ATP-dependent autokinase, CheA, is the crucial step in signal transduction. In vivo, phospho-CheY2 (CheY2-P) is the chief regulator of flagellar rotation, its action being modulated by CheY1 [Sourjik, V., and Schmitt, R. (1996) Mol. Microbiol. 22, 427-436]. In this study, we have investigated these phosphotransfer reactions in vitro using the radiolabeled recombinant proteins, CheA (labeled via [gamma-32P]ATP), CheY1, and CheY2 (labeled via acetyl [32P]phosphate). Our results are consistent with the following four-step phosphotransfer: (i) ATP-dependent autophosphorylation of CheA (with a limiting rate constant of 0.008 s-1 at saturating ATP concentrations); (ii) rapid phospho transfer from phospho-CheA to CheY1 and CheY2; (iii) autodephosphorylation of CheY1-P and CheY2-P with half-lives of 12 +/- 1 s and 10.5 +/- 1 s, respectively; and (iv) reversible phosphotransfer from CheY2-P to CheA. In the three-component mixture, CheA/CheY1/CheY2, we observed rapid phosphotransfer from CheY2-P via CheA to CheY1. Thus, CheY1 assumes the role of a "phosphatase" of CheY2-P by acting as a sink for phosphate, whenever unphosphorylated CheA is present. The intracellular concentrations of CheA/CheY1/CheY2 determined immunochemically were 1.5 microM:20 microM:20 microM, a range that was adopted for in vitro assays. The results reflect a unique control by CheY1 of the active, phosphorylated state of the main response regulator, CheY2-P. This mechanism appears to be a new twist to signal transduction among members of the alpha-subgroup of proteobacteria.

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Rhizobium meliloti MVII-1 and Rhizobium lupini H13-3, two strains with five to ten peritrichously inserted complex flagella, were studied with respect to motility and chemotaxis. Cells of both these strains move rapidly with speeds up to 40 pm s-I (R. meZiZoti) and 60 pm s-' ( R . Zupini) respectively. Increasing viscosity causes little reduction in their swimming velocities as compared with Salmonella typhimurium propelled by plain flagella. It is suggested that complex flagella possess a high 'flexural rigidity', which serves to maintain a helix conformation favourable for propulsive efficiency at increased viscosities. C hemotaxis in R . meliloti MVII-1 and R . lupini H13-3 was studied and the conditions required have been defined using the capillary tube assay. All 20 common L-amino acids and L-homoserine were shown to be attractive to R . meliloti MVII-1 with thresholds varying from lop6 M (proline) to M (aspartate). Leucine, proline and lysine elicited optimal responses. R hizobium lupini H13-3 was also attracted by L-amino acids except for leucine, which elicited no response. Aspartate was a significantly better attractant of R . Zupini H13-3 than of R . meliloti MVII-1, and glycine, isoleucine, homoserine, serine, threonine, cysteine, glutamine and glutamate were