Escherichiu coli like most gram-negative bacteria with walls maintains a cytoplasmic osmolarity exceeding that of the medium ; the resulting hydrostatic pressure (turgor pressure) pushes the cytoplasmic membrane against the peptidoglycan and creates a tension in the two envelopes.Potassium is the only cation wich takes part in the regulation of cellular osmolarity. The adaptation of intracellular K ' concentration to external osmolarity involves K + turgor-controlled fluxes. When the medium osmolarity is raised an osmodependent influx of K can be observed; this is carried out by the K f transport system TrkA which can also taken up rubidium. A specific and unidirectional pathway allows K + ions to flow out of the cell when the medium osmolarity is decreased; this pathway reveals two characteristics: it has no affinity for rubidium and it can be blocked by the blockers of eukaryotic K + channels. Osmodependent fluxes are turned on immediately after the medium osmolarity is disturbed; in contrast, they are turned off gradually as the rate of K t fluxes approach zero. The rate of K' influx seems to depend on the level of internal osmolarity and not on the extent of the increase in medium osmolarity. The rate of the efflux is directly proportional to the decrease in medium osmolarity and is independent on the level of internal osmolarity.Algal cells and higher plant cells have developed osmoregulatory mechanisms in order to withstand osmotic stress and to maintain a constant turgor pressure which is assumed to exert a dual function in initiation of growth and enlargement of cell [l, 21. This regulation is mediated by fluxes of water and organic or inorganic water-soluble molecules and also by the biosynthesis of metabolites of low molecular mass Cells that lack cell walls, such as erythrocytes [4] and certain algae [5], maintain their volume at a constant value over a wide range of external osmotic pressures. Their response to disturbances of the cxternal osmolarity is biphasic [I]; first, water fluxes instantaneously modify the cell volume but not the absolute number of solute molecules, then the cell volume is slowly shifted back to its original value by means of controlled loss or uptake of osmotically active solute molecules and/or stimulation of metabolic turnover of osmoactive solutes. In contrast, walled cells are able to maintain internal osmolarity and pressure higher than in the medium, creating a turgor pressure which is the macroscopic parameter that is regulated during the biphasic response to osmotic stress. The solute transport processes involved in regulation of turgor pressure are identical to those observed in the volume regulation of non-walled-cells [l]. Turgor pressure has been found to be maintained within narrow limits over a wide range of external osmolarities in marine algal cells [I]; its absolute value varies from 2-20 bar according to the species.Bacteria seem to share in some properties of eukaryotic cells since they show a biphasic response to osmotic stress 16, 71 and a turgor pressure...
TheftsZ gene of Escherichia coli, which lies in a cluster of cell division genes at 2 min on the genetic map, codes for a protein which is thought to play a key role in triggering cell division. Using an ftsZ::lacZ operon fusion, we have studied the transcription of the ftsZ gene under conditions in which cell division was either inhibited or synchronized in the bacterial population. InftsZ, ftsA, ftsQ, and ftsl (or pbpB) mutants, there was no change in the differential rate of expression of the ftsZ gene in nonpermissive conditions, when cell division was completely blocked. Although the FtsZ protein is thought to be limiting for cell division, in synchronized cultures theftsZ gene was expressed not only at the moment of septation initiation but throughout the cell cycle. Its expression, however, was not exponential but linear, with a rapid doubling in rate at a specific cell age; this age, about 20 min after division in a 60-min cycle, was different from the age at which the ftsZ::lacZ operon was duplicated. However, it was close to the age at which replication initiated and at which the rate of phospholipid synthesis doubled. During the transient division inhibition after a nutritional shift-up, ftsZ transcription again became linear, with two doublings in rate at intervals equal to the mass doubling time in the rich medium; it adopted the exponential rate typical of rich medium about 60 min after the shift-up, just before the bacterial population resumed cell division. The doubling in the rate of ftsZ transcription once per cycle in synchronized cultures and once per mass doubling time during the transition period after a nutritional shift-up reflects a new cell cycle event.Although a large number of apparent cell division genes have been identified in Escherichia coli and many have been cloned and sequenced, little is known of the precise role of their products (4). The ftsI (or pbpB) gene product, penicillin-binding protein 3, is the only one for which an activity is known (11).Considerable work has been done on one of these division genes, the ftsZ gene, for which a unique temperaturesensitive allele, ftsZ84, provided the demonstration of its role in an early stage of cell division, possibly the initiation of septum formation (22,33). Further evidence for a direct role of the FtsZ protein in septation was provided by the observation (12, 21) that it is the target of the division inhibitor SfiA (or SulA), whose expression is induced by DNA damage as part of the SOS response (9, 10); in this role, FtsZ is a key element of a system coupling cell division to DNA replication.TheftsZ gene lies in a cluster of cell division genes located at 2 min on the E. coli genetic map (22). Its transcriptional organization is complex, with promoters situated in the coding sequences of the adjacentftsA andftsQ genes (29,32,35,36) (see Fig. 1), suggesting possible regulatory interactions among the products of these three genes; certain authors have postulated the existence of a septation complex including all of these protein...
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