The role of lipids in membrane transport in Mycoplasma laidlawii

The comparative metabolism of the mollicutes (Mycoplasmas): the utility for taxonomic classification and the relationship of putative gene annotation and phylogeny to enzymatic function in the smallest free-living cells

Mollicutes or mycoplasmas are a class of wall-less bacteria descended from low G + C% Gram-positive bacteria. Some are exceedingly small, about 0.2 micron in diameter, and are examples of the smallest free-living cells known. Their genomes are equally small; the smallest in Mycoplasma genitalium is sequenced and is 0.58 mb with 475 ORFs, compared with 4.639 mb and 4288 ORFs for Escherichia coli. Because of their size and apparently limited metabolic potential, Mollicutes are models for describing the minimal metabolism necessary to sustain independent life. Mollicutes have no cytochromes or the TCA cycle except for malate dehydrogenase activity. Some uniquely require cholesterol for growth, some require urea and some are anaerobic. They fix CO2 in anaplerotic or replenishing reactions. Some require pyrophosphate not ATP as an energy source for reactions, including the rate-limiting step of glycolysis: 6-phosphofructokinase. They scavenge for nucleic acid precursors and apparently do not synthesize pyrimidines or purines de novo. Some genera uniquely lack dUTPase activity and some species also lack uracil-DNA glycosylase. The absence of the latter two reactions that limit the incorporation of uracil or remove it from DNA may be related to the marked mutability of the Mollicutes and their tachytelic or rapid evolution. Approximately 150 cytoplasmic activities have been identified in these organisms, 225 to 250 are presumed to be present. About 100 of the core reactions are graphically linked in a metabolic map, including glycolysis, pentose phosphate pathway, arginine dihydrolase pathway, transamination, and purine, pyrimidine, and lipid metabolism. Reaction sequences or loci of particular importance are also described: phosphofructokinases, NADH oxidase, thioredoxin complex, deoxyribose-5-phosphate aldolase, and lactate, malate, and glutamate dehydrogenases. Enzymatic activities of the Mollicutes are grouped according to metabolic similarities that are taxonomically discriminating. The arrangements attempt to follow phylogenetic relationships. The relationships of putative gene assignments and enzymatic function in My. genitalium, My. pneumoniae, and My. capricolum subsp. capricolum are specially analyzed. The data are arranged in four tables. One associates gene annotations with congruent reports of the enzymatic activity in these same Mollicutes, and hence confirms the annotations. Another associates putative annotations with reports of the enzyme activity but from different Mollicutes. A third identifies the discrepancies represented by those enzymatic activities found in Mollicutes with sequenced genomes but without any similarly annotated ORF. This suggests that the gene sequence is significantly different from those already deposited in the databanks and putatively annotated with the same function. Another comparison lists those enzymatic activities that are both undetected in Mollicutes and not associated with any ORF. Evidence is presented supporting the theory that there are relatively small gene sequences that code for functional centers of multiple enzymatic activity. This property is seemingly advantageous for an organism with a small genome and perhaps under some coding restraint. The data suggest that a concept of "remnant" or "useless genes" or "useless enzymes" should be considered when examining the relationship of gene annotation and enzymatic function. It also suggests that genes in addition to representing what cells are doing or what they may do, may also identify what they once might have done and may never do again.

Heat stress induces a glycosylation of membrane sterol in myxoamoebae of a true slime mold, Physarum polycephalum

To know the very early events occurring after heat shock, the changes of membrane lipids were examined. Heat stress induced the production of a certain glycolipid in the myxoamoebae of Physarum polycephalum in a few minutes. The purified glycolipid was determined to be a poriferasterol monoglucoside by structural studies that was previously reported to be expressed during the differentiation of Physarum cells from haploid myxoamoebae to diploid plasmodia (Murakami-Murofushi, K., Nakamura, K., Ohta, J., Suzuki, M., Suzuki, A., Murofushi, H., and Yokota, T. (1987) J. Biol. Chem. 262, 16719-16723). The activity of UDP-glucose:poriferasterol glucosyltransferase (Murakami-Murofushi, K., and Ohta, J. (1989) Biochim. Biophys. Acta 992, 412-415) was also expressed rapidly after heat shock. Thus, the activation of sterol glucosyltransferase and the production of sterol-glucoside were considered to be important events that were involved in the signal transduction system to induce some succeeding heat-shock responses, such as the synthesis of heat-shock proteins.

Expression of UDP-glucose: poriferasterol glucosyltransferase in the process of differentiation of a true slime mold, Physarum polycephalum

An expression of UDP-glucose:poriferasterol glucosyltransferase activity associated with differentiation of a true slime mold, Physarum polycephalum, from haploid myxoamoebae to diploid plasmodia was demonstrated. In the haploid cells, this enzyme activity was not detected, but after conjugation of the myxoamoebae, the enzyme activity was expressed and increased definitely. In the plasmodial stage, high enzyme activity was maintained constantly. The enzyme was partially purified (35-fold purification, and 28% yield), and molecular weight of 72,000, pH optimum of 7.0, and some characteristics were demonstrated.

Biosynthesis of cholesteryl glucoside by Mycoplasma gallinarum

The biosynthesis of cholesteryl glucoside by Mycoplasma gallinarum strain J proceeds by the transfer of glucose from uridine-5'-diphosphoglucose to membrane-bound sterol. Galactose also can be coupled to cholesterol via uridine-5'-diphosphogalactose. The reaction is specific for the uridine-5'-diphospho sugars. Enzymatic activity is associated with the membrane. Treatment of the membrane to remove endogenous sterol inactivates the enzyme. Only sterol which has been bound to the membrane participates in the reaction. The optimum pH is about 8.0, and Mg(2+) is required. The reaction is unaffected by nucleotide triphosphate, uridine-5'-monophosphate, and uridine-5'-diphosphate. Reduction of pH to the optimum for beta-glucosidase in the membrane results in loss of synthesized glucoside. The enzyme is saturated at 0.5 mm uridine-5'-diphosphoglucose. The apparent K(m) of 2.05 x 10(-7) indicates a high affinity of the enzyme for the nucleotide sugar.

Changes in membrane lipid composition in exponentially growing Staphylococcus aureus during the shift from 37 to 25 C

Lowering the temperature of growth of Staphylococcus aureus from 37 to 25 C decreased the growth rate and induced changes in the composition of the membrane lipids. Changes in lipid composition also occur in the transition between exponential and stationary growth phases at one temperature. To isolate the effects of lowering the temperature, exponentially growing S. aureus was abruptly switched from 37 to 25 C by transfer to cooler medium. Exponential growth continued at 25 C without a lag period but with a threefold increase in doubling time. In the period of exponential growth at suboptimal temperature, there was essentially no change in the fatty acid composition of the lipids, little change in the vitamin K(2) composition with perhaps a slight increase in the total level, and essentially no change in the phospholipid composition, but a marked stimulation of the synthesis of the rubixanthins. Growth of cells at 25 C was much more sensitive to the inhibition of rubixanthin formation by mixed-function oxidase inhibitors than cells growing at 37 C, suggesting some function for the rubixanthins at suboptimal temperatures. The striking increases in the proportions of monoenoic fatty acids observed at lowered growth temperatures in many biological systems are not detected in S. aureus.

Metabolism of the glycosyl diglycerides and phosphatidylglucose of Staphylococcus aureus

A glucose containing lipid, phosphatidylglucose (probably 3-sn-phosphatidyl-1'-glucose) and a lipid tentatively identified as phosphatidylethanolamine have been characterized in the lipids of Staphylococcus aureus. These lipids together comprise less than 2% of the total phospholipids of exponentially growing S. aureus and accumulate to 14% of the total phospholipid in stationary-phase cells. These lipids lost no (32)P when cells grown with H(3) (32)PO(4) were transferred to nonradioactive medium during the exponential growth phase. This was in marked contrast to the other phospholipids which lost (32)P rapidly. The loss of (32)P from phosphatidic acid and cardiolipin in exponentially growing cells was biphasic, suggesting heterogeneity of phospholipid phosphate metabolism. The mono- and diglucosyl diglycerides showed a rapid loss of (14)C-glucose during growth in nonradioactive medium but no loss of (14)C from the fatty acids of these lipids. The (14)C in the glucose and fatty acids of the glucosyl diglycerides was derived from glucose.