Glutamate and cyclic AMP regulate the expression of galactokinase in Mycobacterium smegmatis

It was found that Mycobacterium smegmatis is unable to utilize galactose as the sole carbon source because the sugar alone cannot induce galactokinase. However, galactokinase was induced by glutamate alone, and was further stimulated by galactose. Rifampicin completely inhibited the glutamate-mediated expression of galK in both the absence and presence of galactose. Extracellular cAMP stimulated the expression of the enzyme only in the presence of glutamate plus galactose. The galK gene from M. smegmatis, including its upstream promoter region, was cloned in a plasmid in Escherichia coli. The expression of kinase from these clones in E. coli was dependent on cAMP and its receptor protein (CRP). The expression of UDP-galactose 4-epimerase was constitutive. This and other evidence suggests that the galK gene is not linked to galT and galE in the mycobacterial genome. In a glutamate-independent galactose-utilizing mutant (gin-1 mutant) of M. smegmatis, galK was expressed in the absence of both galactose and glutamate, while in the presence of galactose this expression was increased twofold in the absence of glutamate and fourfold in its presence. Extracellularly added cAMP reduced the expression of the enzyme in the presence of galactose plus glutamate nearly to the basal level. It is proposed that in M. smegmatis the galK gene is expressed from two different promoters; the expression from one promoter is dependent on glutamate but not on galactose and cAMP, while that from the other requires all three components. The role of galactose is possibly to derepress the latter promoter

Bacteriophage λ p gene shows host killing which is not dependent on λ DNA replication

Bacteriophage λ having a mutation replacing glycine by glutamic acid at the 48th codon of cro, kills the host under N- conditions; we call this the hk mutation. In λN-N-cl-hk phage-infected bacteria, the late gene R i expressed to a significant level, phage DNA synthesis occurs with better efficiency, and the Cro activity is around 20% less, all compared to those in λN-N-cl-hk+-infected bacteria. Segments of λ DNA from the left of pR to the right of tR2, carrying cro, cll, O,P, and the genes of the nin5 region from the above hk and hk+ phages, were cloned in pBR322. Studies with these plasmids and their derivatives having one or more of the λ genes deleted indicate that the hk mutation is lethal only when a functional P gene is also present. When expression of P from pR is elevated, due to the deletion of tR1, host killing also occurs without the hk mutation. We conclude that the higher levels of P protein, produced either (1) when cro has the hk mutation or (2) when tR1 is deleted, are lethal to the host. We also show that due to the hk mutation, the Cro protein becomes partially defective in its negative regulation at pR, resulting in the expression of P to a lethal level even in the absence of N protein-mediated antitermination. This P protein-induced host killing depends neither on λ DNA replication nor on any other gene functions of the phage

Multiphasic denaturation of the λ repressor by urea and its implications for the repressor structure

Urea denaturation of the λ repressor has been studied by fluorescence and circular dichroic spectroscopies. Three phases of denaturation could be detected which we have assigned to part of the C-terminal domain, N-terminal domain and subunit dissociation coupled with further denaturation of the rest of the C-terminal domain at increasing urea concentrations. Acrylamide quenching suggests that at least one of the three tryptophan residues of the λ repressor is in a different environment and its emission maximum is considerably blue-shifted. The transition in low urea concentration (midpoint approximately 2 M) affects the environment of this tryptophan residue, which is located in the C-terminal domain. Removal of the hinge and the N-terminal domain shifts this transition towards even lower urea concentrations, indicating the presence of interaction between hinge on N-terminal and C-terminal domains in the intact repressor

Repressor of temperate mycobacteriophage L1 harbors a stable C-terminal domain and binds to different asymmetric operator DNAs with variable affinity

<p>Abstract</p> <p>Background</p> <p>Lysogenic mode of life cycle of a temperate bacteriophage is generally maintained by a protein called 'repressor'. Repressor proteins of temperate lambdoid phages bind to a few symmetric operator DNAs in order to regulate their gene expression. In contrast, repressor molecules of temperate mycobacteriophages and some other phages bind to multiple asymmetric operator DNAs. Very little is known at present about the structure-function relationship of any mycobacteriophage repressor.</p> <p>Results</p> <p>Using highly purified repressor (CI) of temperate mycobacteriophage L1, we have demonstrated here that L1 CI harbors an N-terminal domain (NTD) and a C-terminal domain (CTD) which are separated by a small hinge region. Interestingly, CTD is more compact than NTD at 25°C. Both CTD and CI contain significant amount of α-helix at 30°C but unfold partly at 42°C. At nearly 200 nM concentration, both proteins form appreciable amount of dimers in solution. Additional studies reveal that CI binds to <it>O</it>₆₄and <it>O</it>_{<it>L </it>}types of asymmetric operators of L1 with variable affinity at 25°C. Interestingly, repressor – operator interaction is affected drastically at 42°C. The conformational change of CI is most possibly responsible for its reduced operator binding affinity at 42°C.</p> <p>Conclusion</p> <p>Repressors encoded by mycobacteriophages differ significantly from the repressor proteins of λ and related phages at functional level but at structural level they are nearly similar.</p

Cloning and characterization of the promoters of temperate mycobacteriophage L1

Four putative promoters of the temperate mycobacteriophage L1 were cloned by detecting the β-galactosidase reporter expression in E. coli transformants that carried L1 specific operon-fusion library. All of the four L1 promoters were also found to express differentially in the homologous environment of mycobacteria. Of the four promoters, two were suggested to be the putative early promoters of L1 since they express within 0 to 10 min of the initiation of the lytic growth of L1. One of the putative early promoters showed a relatively better and almost identical activity in both E. coli and M. smegmatis. By a sequence analysis, we suggest that the L1 insert that contained the stronger early promoter possibly carries two convergent E. coli δ 70-like L1 promoters, which are separated from each other by about 300 nucleotides. One of them is the early promoter of L1 as it showed a 100% similarity with the early Pleft promoter of the homoimmune phage L5. The second promoter, designated P4, was suggested for its appreciable level of reporter activity in the absence of the -10 element of the Pleft equivalent of L1. By analyzing most of the best characterized mycobacteriophages-specific promoters, including the L1 promoter P4, we suggest that both the -10 and -35 hexamers of the mycobacteriophage promoters are highly conserved and almost similar to the consensus -10 and -35 hexamers of the E. coli δ 70 promoters

Role of the C-terminal tail region in the self-assembly of λ-repressor

Acrylamide quenching of the tryptophan fluorescence of the λ-repressor at different protein concentrations indicates that one of the three tryptophan residues, W129, W142, and W230, undergoes a change in environment upon self-assembly, from dimer to associated species. Quenching data suggest that this tryptophan residue is inaccessible to low concentrations of acrylamide and is blue-shifted in the associated form. In the dimer, this tryptophan residue is highly accessible to acrylamide and is red-shifted. NBS oxidation, at protein concentrations which favor the associated form, showed that this tryptophan is also significantly protected from NBS oxidation. HPLC peptide mapping of NBS-oxidized λ-repressor, amino acid analysis, and sequencing indicate that the protected, blue-shifted tryptophan is tryptophan 230. A mutant repressor (F235C) was specifically labeled at Cys 235 with an environment-sensitive probe, acrylodan. The acrylodan fluorescence of the labeled F235C λ -repressor undergoes a significant blue-shift, accompanied by fluorescence enhancement, upon protein association. Along with other genetic evidence, these results suggest involvement of the C-terminal tail region in the self-assembly of the λ-repressor

The Bacteriophage λ DNA replication protein P inhibits the oriC DNA-and ATP-binding functions of the DNA replication initiator protein DnaA of escherichia coli

Under the condition of expression of λ P protein at lethal level, the oriC DNA-binding activity is significantly affected in wild-type E. coli but not in the rpl mutant. In purified system, the λ P protein inhibits the binding of both oriC DNA and ATP to the wild-type DnaA protein but not to the rpl DnaA protein. We conclude that the λ P protein inhibits the binding of oriC DNA and ATP to the wild-type DnaA protein, which causes the inhibition of host DNA synthesis initiation that ultimately leads to bacterial death. A possible beneficial effect of this interaction of λ P protein with E. coli DNA initiator protein DnaA for phage DNA replication has been proposed

A study of energetics of cooperative interaction using a mutant λ-repressor

A &#955;-repressor mutant, S228N, which is defective in tetramer formation in the free state but retains full cooperativity, was studied in detail. Isolated single operator-bound S228N repressor shows association properties similar to those of the wild-type repressor. Fluorescence anisotropy studies with dansyl chloride-labeled repressor show a dimer-monomer dissociation constant of around 10-5 M. The structure of the mutant repressor was studied by circular dichroism, acrylamide quenching and sulfhydryl reactivity at protein concentrations of &#8804; 10-6 M, where it is predominantly monomeric. The results suggest no significant perturbations in the structure of the S228N mutant repressor from that of the wild-type repressor. Urea denaturation studies also indicate no significant change in the stability of the repressor. The results were used to calculate energetics of loop formation in the cooperative binding process

A point mutation at the C-terminal half of the repressor of temperate mycobacteriophage L1 affects its binding to the operator DNA

The wild-type repressor CI of temperate mycobacteriophage L1 and the temperature-sensitive (ts) repressor CIts391 of a mutant L1 phage, L1cIts391, have been separately overexpressed in E. coli. Both these repressors were observed to specifically bind with the same cognate operator DNA. The operator-binding activity of CIts391 was shown to differ significantly than that of the CI at 32 to 42° C. While 40-95% operator-binding activity was shown to be retained at 35 to 42° C in CI, more than 75% operator-binding activity was lost in CIts391 at 35 to 38° C, although the latter showed only 10% less binding compared to that of the former at 32° C. The CIts391 showed almost no binding at 42° C. An in vivo study showed that the CI repressor inhibited the growth of a clear plaque former mutant of the L1 phage more strongly than that of the CIts391 repressor at both 32 and 42° C. The half-life of the CIts391-operator complex was found to be about 8 times less than that of the CI-operator complex at 32° C. Interestingly, the repressoroperator complexes preformed at 0oC have shown varying degrees of resistance to dissociation at the temperatures which inhibit the formation of these complexes are inhibited. The CI repressor, but not that of CIts391, regains most of the DNA-binding activity on cooling to 32° C after preincubation at 42 to 52° C. All these data suggest that the 131st proline residue at the C-terminal half of CI, which changed to leucine in the CIts391, plays a crucial role in binding the L1 repressor to the cognate operator DNA, although the helix-turn-helix DNA-binding motif of the L1 repressor is located at its N-terminal end