Adaptation of the cells of Escherichia coli to the presence of hydroxyurea increases the level of inorganic pyrophosphatase acttivity

The chromosome cycle of prokaryotes

In both eukaryotes and prokaryotes, chromosomal DNA undergoes replication, condensation-decondensation and segregation, sequentially, in some fixed order. Other conditions, like sister-chromatid cohesion (SCC), may span several chromosomal events. One set of these chromosomal transactions within a single cell cycle constitutes the 'chromosome cycle'. For many years it was generally assumed that the prokaryotic chromosome cycle follows major phases of the eukaryotic one: -replication-condensation-segregation-(cell division)-decondensation-, with SCC of unspecified length. Eventually it became evident that, in contrast to the strictly consecutive chromosome cycle of eukaryotes, all stages of the prokaryotic chromosome cycle run concurrently. Thus, prokaryotes practice 'progressive' chromosome segregation separated from replication by a brief SCC, and all three transactions move along the chromosome at the same fast rate. In other words, in addition to replication forks, there are 'segregation forks' in prokaryotic chromosomes. Moreover, the bulk of prokaryotic DNA outside the replication-segregation transition stays compacted. I consider possible origins of this concurrent replication-segregation and outline the 'nucleoid administration' system that organizes the dynamic part of the prokaryotic chromosome cycle.

Comparative kinetic studies on the two interconvertible forms of Streptococcus faecalis inorganic pyrophosphatase

In this work the two interconvertible forms of inorganic pyrophosphatase (EC 3.6.1.1) of Streptococcus faecalis were shown to differ in kinetics. The highly active form of the enzyme was more sensitive to the changes in the Mg2+ concentration, and thus also more sensitive to the inhibition caused by ATP, which competes with PPi for the chelation of Mg2+ ions. We have previously described a kinetic model for the less-active form of S. faecalis inorganic pyrophosphatase [Lahti & Jokinen (1985) Biochemistry 24, 3526-3530]. The kinetic model of the highly active enzyme form is proposed to be a modification of the model of the less-active form in which enzyme activation by free Mg2+ is necessary for the reaction to occur. In this model the enzyme exists in two states, referred to as R- and T-states. In the absence of ligands the enzyme is in the T-state. R-state, i.e. the catalytically active state, exists only in the presence of free Mg2+. Mg1PPi2- is the primary substrate, and free pyrophosphate is a weak inhibitor that cannot serve as a substrate for the highly active form of S. faecalis inorganic pyrophosphatase. This model closely resembles that previously presented for yeast inorganic pyrophosphatase.

Diphosphate concentration does not correlate with the level of inorganic diphosphatase in Escherichia coli

Partial inhibition of DNA synthesis stimulates the production of inorganic diphosphatase in Escherichia coli but the changes in diphosphate (PPi) level observed did not correlate with the enzyme activity. An accumulation of PPi was observed in the presence of inhibitors of RNA synthesis or nucleotide synthesis. In the former case the level of the enzyme did not change but in the latter case it increased. Thus the amount of inorganic diphosphatase alone does not determine the concentration of PPi in E. coli.

Regulation of inorganic pyrophosphatase in Escherichia coli: relationship between the synthesis of inorganic pyrophosphatase and the thymidine triphosphate pool

The activities of inorganic pyrophosphatase, thymidine kinase and thymidine phosphorylase were measured in Ter-mutants of E. coli K12 which have a higher or a lower dTTP pool than the parent strain. The levels of inorganic pyrophosphatase and thymidine kinase were changed in the same direction and that of thymidine phosphorylase in the opposite direction in these mutants.