In a lysogen, most genes of phage lambda are repressed; in response to a transient induction signal, they are efficiently switched on. The switch, which consists in part of a tripartite operator to which two regulatory proteins bind, depends not only on DNA--protein interactions, but also on effects transmitted from one DNA-bound protein to another. lambda Exemplifies a strategy that facilitates efficient switching between two physiological states in response to a transient signal.

No abstract

We have cloned and sequenced the gene for DNA ligase from Thermus thermophilus. A comparison of this sequence and those of other ligases reveals significant homology only with that of Escherichia coli. The overall amino acid composition of the thermophilic ligase and the pattern of amino acid substitutions between the two proteins are consistent with compositional biases in other thermophiic enzymes. We have engineered the expression of the T. thermophilus gene in Escherichia coli, and we show that E. coli proteins may be substantially removed from the thermostable ligase by a simple heat precipitation step.DNA ligases (EC 6.5.1.1 or 6.5.1.2) are essential enzymes in the replication, recombination, and repair of DNA. As such, we might expect that an archetypical ligase arose at an early point in evolution and then was disseminated throughout the biota. Several points stand in contrast to such a view. The sequences of ligases from eukaryotes (5-7), prokaryotes (22), bacteriophages (1, 17), and viruses (39) (18,40). In the first step, the AMP moiety of a high-energy adenylate (NAD or ATP) is covalently linked to the enzyme, releasing NMN or pyrophosphate. Next, this AMP moiety is transferred to the phosphorylated 5' end of a DNA strand, creating a new diphosphate linkage. Finally, the 3' hydroxyl of an adjacent DNA strand attacks the new diphosphate linkage, which releases AMP, creates a phosphodiester linkage, and thereby completes the covalent joining of the DNA strands. Either the various ligases are truly homologous in a way which is not readily apparent at the primary sequence level (but exists at a secondary or tertiary structure level), or they have convergently evolved identically detailed reaction mechanisms.Thermus thermophilus determines a DNA ligase that is active at temperatures above 75°C (42). This enzyme is present in low abundance, and its purification (42) is complicated by the presence of host proteases. Although it is known to utilize NAD, its relatedness to Escherichia coli DNA ligase was unexplored. We wished to obtain greater quantities of this enzyme free from thermostable contaminants, and we wished to understand its phylogenetic rela-

No abstract

No abstract