Development Of Myxococcus Xanthus Research Articles

Gene expression during development of Myxococcus xanthus: Analysis of the genes for protein S

Protein S is an abundant spore coat protein produced during fruiting body formation (development) of the bacterium Myxococcus xanthus. We have cloned the DNA which codes for protein S and have found that this DNA hybridizes to three protein S RNA species from developmental cells but does not hybridize to RNA from vegetative cells. The half-life of protein S RNA was found to be unusually long, about 38 minutes, which, at least in part, accounts for the high level of protein S synthesis observed during development. Hybridization of restriction fragments from cloned M. xanthus DNA to the developmental RNAs enabled us to show that M. xanthus has two directly repeated genes for protein S (gene 1 and gene 2) which are separated by about 10 3 base-pairs on the bacterial chromosome. To study the expression of the protein S genes in M. xanthus, eight M. xanthus strains were isolated with Tn5 insertions at various positions in the DNA which codes for protein S. The strains which contained insertions in gene 1 or between gene 1 and gene 2 synthesized all three protein S RNA species and exhibited normal levels of protein S on spores. In contrast, M. xanthus strains with insertions in gene 2 had no detectable protein S on spores and lacked protein S RNA. Thus, gene 2 is responsible for most if not all of the production of protein S during M. xanthus development. M. xanthus strains containing insertions in gene 1, gene 2 or both genes, were found to aggregate and sporulate normally even though strains bearing insertions in gene 2 contained no detectable protein S. We examined the expression of gene 1 in more detail by constructing a fusion between the lacZ gene of Escherichia coli and the N-terminal portion of protein S gene 1 of M. xanthus. The expression of β-galactosidase activity in an M. xanthus strain containing the gene fusion was shown to be under developmental control. This result suggests that gene 1 is also expressed during development although apparently at a much lower level than gene 2.

Cell-Cell Interactions and Development in Myxococcus Xanthus

Myxococcus xanthus is a gram-negative, rod-shaped bacterium which shows complex cell-cell interactions during vegetative growth and fruiting-body formation. In nature, Myxococcus xanthus grows and moves as a large community of food gathering and predatory organisms. Cell-cell contact is maintained throughout vegetative growth and characteristic behaviors are exhibited in the presence of certain stimuli. In addition, cell-cell stimulation can be observed in the movement of cells. Thus there must exist specific chemical signals that help direct the movement of individual cells within the community. The most dramatic behaviors exhibited by Myxococcus xanthus are associated with starvation. Starved cells aggregate and then sporulate within raised mounds. Complex biochemical changes which are part of a developmental program occur in these cells. For example, protein S first appears in the cytoplasmic fraction of cells, and is later translocated to the cell surface where it self-assembles to form a spore surface coat. Another new developmental protein that is synthesized at this time is myxobacterial hemagglutinin, a lectin that accumulates in the shockable fraction of developmental cells and that has been located in distinct patches at the cell poles. This protein may be important in end-to-end associations during cellular aggregation. Cell-cell signalling has also been found to be important during fruiting-body formation. Certain non-fruiting mutants can be stimulated to fruit by wild-type cells or by mutants from a different extracellular complementation group. Analysis of mutants has shown that the developmental program in Myxococcus xanthus appears to have two major components: one controls aggregation; the other regulates sporulation. Links exist between these two components, since some developmental mutants are defective in both functions. The study of cell-cell interactions during vegetative growth and fruiting-body formation in M. xanthus provides an opportunity to analyze complex interactions in a simple model system.

Synthesis of several membrane proteins during developmental aggregation in Myxococcus xanthus.

We have examined the pattern of synthesis of several membrane proteins during the aggregation phase of development in Myxococcus xanthus. Development was initiated by plating vegetative cells on polycarbonate filters placed on top of an agar medium that supported fruiting body formation. At various times during aggregation a filter was removed, the cells were pulse-labeled with [35S]methionine, and the membrane proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The rate of synthesis of numerous individual proteins changed during aggregation; we concentrated on six whose pattern of synthesis was greatly altered during aggregation. The rate of synthesis of five of the six proteins increased considerably during aggregation; that of the remaining protein was curtailed and appeared to be regulated by nutrient conditions. Three of the five major membrane proteins that increased during aggregation had a unique pattern of synthesis that was displayed only under conditions that are are required for development - high cell density, nutrient depletion, and a solid (agar) surface. The remaining two proteins were not unique to development; the appearance of one protein could be induced under conditions of high cell density, whereas the other could be induced by placing the cells on a solid agar surface. All of the five major proteins that appeared during development did so during the preaggregation stage, and the synthesis of four of the five proteins appeared to be curtailed late in aggregation. The synthesis of the remaining protein continued throughout aggregation.

Cyclic adenosine 3',5'-monophosphate binding protein in developing myxospores of Myxococcus xanthus.

The interaction of cyclic adenosine 3',5'-monophosphate (cAMP) with specific protein molecules was examined in the high-speed supernatant fraction of extracts made at stages throughout glycerol-induced myxospore development in Myxococcus xanthus. Experiments using 8-azido[32P]cAMP, a photoaffinity analogue of cAMP, and SDS - polyacrylamide gel electrophoresis showed that the nucleotide interacts with only a single protein band of 12 500 molecular weight. Both the identiy and amount of this protein remained constant throughout development. The binding protein was specific for cAMP; other nucleotides did not compete with cAMP for binding sites. A Scatchard analysis showed evidence of only a single class of binding sites with a high affinity for cAMP.

Sequential changes in the cyclic nucleotide levels and cyclic nucleotide phosphodiesterase activities during development ofmyxococcus xanthus.

The levels of adenosine 3',5'-monophosphate, guanosine 3'-5'-monophosphate and the activities of their respective phosphodiesterases exhibited changes during development ofMyxococcus xanthus that are substantially consistent with role postulated for each in a previously proposed model.

Gene expression during development of Myxococcus xanthus: Pattern of protein synthesis

The pattern of protein synthesis during development of Myxococcus xanthus was investigated. This gram-negative bacterium has a complex life cycle which involves a temporal sequence of cellular aggregation, mound formation, and myxosporulation. At various stages of development, cells were pulse-labeled with a 14C-labeled amino acid mixture. Synthesis of soluble and membrane proteins was then analyzed by SDS-polyacrylamide gel electrophoresis. Of the 30 major soluble proteins, at least 25% showed significant changes in their rates of production during development. Several significant changes were also found in the membrane proteins as analyzed by two-dimensional polyacrylamide gel electrophoresis. The major proteins synthesized during development were classified into four different types: accumulation proteins, peak proteins, late proteins, and constant proteins. The synthesis of protein S, an accumulation protein, increases dramatically during development to a maximum of 15% of total soluble protein synthesis. When methionine was added to the culture medium, cells did not form fruiting bodies. Under these conditions, almost all of the protein changes observed in the early and middle periods of development still occurred. However, the production of late proteins (e.g., protein U) was not observed, suggesting that methionine blocks a late stage of development. During glycerol induction, many of the changes in protein synthesis which normally occur during development were not observed (e.g., protein S did not accumulate). These results indicate that gene expression in M. xanthus is complex and subject to tight regulation.

Rifampin-resistant mutants of Myxococcus xanthus defective in development.

Rifampin, an antibiotic which is known to bind to and inhibit RNA polymerase, was used to probe the molecular regulation of development in Myxococcus xanthus. Rifampin-resistant mutants were screened for defects in fruiting-body formation. About 20% of the isolates in the initial screenings showed major defects in developmental aggregation or sporulation. Eleven independent mutants with wild-type growth rates and stable phenotypes were analyzed by transduction. In these strains, the rifampin-resistant and nonfruiting phenotypes showed cotransduction frequencies equal to or greater than 99.0 to 99.9%. The RNA polymerase activities were resistant to rifampin in vitro, indicating that the RNA polymerase is altered in these strains. Although their fruiting phenotypes are heterogeneous, these strains can be divided into two classes based on the level of aggregation. The results suggest that RNA polymerase plays a significant role in the regulation of development in M. xanthus since mutations which cause no apparent changes in vegetative growth result in striking defects in fruiting-body formation.

Changes in cyclic AMP levels during development in Myxococcus xanthus

Cyclic AMP levels doubled in Myxococcus xanthus under conditions in which cells aggregate and form fruiting bodies. In liquid medium, glycerol- or dimethyl sulfoxide-induced sporulating cultures exhibited a sharp but transient rise in cyclic AMP concentration after 45 min.

Regulation of development in Myxococcus xanthus: effect of 3':5'-cyclic AMP, ADP, and nutrition.

An assay was developed to study the regulation of fruiting in Myxococcus xanthus. The nucleotides, adenosine 3':5'-cyclic monophosphate (cyclic AMP) and adenosine diphosphate (ADP), were found to greatly stimulate fruiting under the assay conditions. Very sharp concentration optima were observed. Even under conditions of starvation, these nucleotides greatly increased the number of aggregation sites. Nutrition was found to influence fruiting body morphology. The effect of amino acids on the nucleotide stimulation of fruiting was studied under our assay conditions. L-Methionine and L-isoleucine (1 mM) completely blocked either L-threonine or D,L-diaminopimelic acid synergistically enhanced the amount of fruiting in the presence of these nucleotides. The data presented suggest the existence of differentiation-related regulatory compounds in M. xanthus.

Changes in activity of glyoxylate cycle enzymes during myxospore development in Myxococcus xanthus.

Activities of the glyoxylate cycle enzymes isocitrate lyase (EC 4.1.3.1) and malate synthase (EC 4.1.3.2) were assayed in extracts prepared at different stages of myxospore formation in liquid cultures of Myxococcus xanthus. Activities of both enzymes attained peak values during conversion of rods to spheres. Isocitrate lyase activity decreased after reaching its peak value. Malate synthase activity also declined but at a much slower rate. The loss of isocitrate lyase activity could be prevented by the addition of chloramphenicol to cultures early in myxospore formation (during the initial rise in enzyme activity), but not by such addition at later stages of myxospore formation. The increase in glyoxylate cycle enzymes was not observed in a mutant unable to form myxospores in liquid culture under conditions suitable for morphological conversion of the wild type, or in wild-type cells incubated in the absence of an inducer for myxospore formation. It is concluded that the changes in the glyoxylate cycle enzymes represent regulatory phenomena associated with the development of the myxospore.