Genetic and Physical Mapping of the Mcra (Rgla) and Mcrb (Rglb) Loci of Escherichia Coli K-12

We have genetically analyzed, cloned and physically mapped the modified cytosine-specific restriction determinants mcrA (rglA) and mcrB (rglB) of Escherichia coli K-12. The independently discovered Rgl and Mcr restriction systems are shown to be identical by three criteria: 1) mutants with the RglA- or RglB- phenotypes display the corresponding McrA- or McrB- phenotypes, and vice versa; 2) the gene(s) for RglA and McrA reside together at one locus, while gene(s) for RglB and McrB are coincident at a different locus; and 3) RglA+ and RglB+ recombinant clones complement for the corresponding Mcr-deficient lesions. The mcrA (rglA) gene(s) is on the excisable element e14, just clockwise of purB at 25 min. The mcrB (rglB) gene(s), at 99 min, is in a cluster of restriction functions that includes hsd and mrr, determinants of host-specific restriction (EcoK) and methyladenine-specific restriction respectively. Gene order is mcrB-hsdS-hsdM-hsdR-mrr-serB. Possible models for the acqusition of these restriction determinants by enteric bacteria are discussed.     

Enzyme catalyzed hydrolysis of inorganic pyrophosphate. Current view of problems in characterization of PP1-phosphohydrolytic activity associated with alkaline phosphatases (EC 3.1.3.1).

A survey of the hydrolytic activity of alkaline phosphatase (EC 3.1.3.1) reveals that PP1, like phosphomonoesters, can serve as substrate in vitro. This pp1-phosphohydrolytic activity can be distinguished from PP1-phosphohydrolytic activities of inorganic pyrophosphatases (EC 3.6.1.1) and glucose-6-phosphatase (EC 3.1.3.9) by several criteria. Discrimination among these hydrolytic enzymes is possible by their dependence on variation of pH and of magnesium to PP1 ratios in the assay solutions. The true substrates and modifiers are not simply PP1 and magnesium, but the equilibrium species in mixtures of these two. The physiological significance of each of the three enzymes is not predictable from their differential efficiency as catalysts of PP1-hydrolysis in vitro.