A mutant of Bacillus thuringiensis var. israelensis (B.t.i.) resistant to antibiotics

Summary PST, a spontaneous mutant of Bacillus thuringiensis var. israelensis (B.t.i.) resistant to penicillin, streptomycin and tetracycline was isolated by serial selections. In the absence of antibiotics it showed genetic stability for 16 generations. Mosquito larvicidal activity of PST was similar to that of B.t.i., its parental strain. It also maintained the specific antigenicity of B.t.i. although its rate of growth was somewhat lower, a generation time of 55 min for PST vs. 38 min for B.t.i. Cell concentration plays a major role in the phenomenon of PST resistance to penicillin.This antibiotic resistant mutant of b.t.i. provides us with an efficient tool to trace B.t.i. among the indigenous bacteria present in septic habitats in the field as well as inside the larval gut.     

Relationships among the herbicide and functional sites of acetohydroxy acid synthase from Chlorella emersonii

The properties of acetohydroxy acid synthase (AHAS, EC 4.1.3.18) from wild-type Chlorella emersonii (var. Emersonii, CCAP-211/11n) and two spontaneous sulfometuron methyl (SMM)-resistant mutants were examined. The AHAS from both mutants was resistant to SMM and cross-resistant to imazapyr (IM) and the triazolopyrimidine sulfonanilide herbicide XRD-498 (TP). The more-SMM-resistant mutant had AHAS with altered catalytic parameters (K _m, specificity), but unchanged sensitivity to the feedback inhibitors valine and leucine. The second mutant enzyme was less sensitive to the feedback inhibitors, but had otherwise unchanged kinetic parameters. Inhibition-competition experiments indicated that the three herbicides (SMM, IM, TP) bind in a mutually exclusive manner, but that valine can bind simultaneously with SMM or TP. The three herbicide classes apparently bind to closely overlapping sites. We suggest that the results with C. emersonii and other organisms can all be explained if there are separate binding sites for herbicides, feedback inhibitors and substrates.Abbreviations AHAS acetohydroxy acid synthase - AL acetolactate - AHB acetohydroxybutyrate - IM imazapyr - TP triazolopyrimidine sulfonanilide herbicide XRD-498 - R enzyme specificity - SMM sulfometuron methyl This research was supported in part by the United States — Israel Binational Science Foundation (BSF), Jerusalem, Israel (Grant 86-00205) and the Fund for Basic Research, Israel Academy of Sciences.     

Inhibition of melanoma development in the Nras(Q61K)::Ink4a−/− mouse model by the small molecule BI‐69A11

To date, there are no effective therapies for tumors bearing NRAS mutations, which are present in 15–20% of human melanomas. Here we extend our earlier studies where we demonstrated that the small molecule BI‐69A11 inhibits the growth of melanoma cell lines. Gene expression analysis revealed the induction of interferon‐ and cell death‐related genes that were associated with responsiveness of melanoma cell lines to BI‐69A11. Strikingly, the administration of BI‐69A11 inhibited melanoma development in genetically modified mice bearing an inducible form of activated Nras and a deletion of the Ink4a gene (Nras^(Q61K)::Ink4a^?/?). Biweekly administration of BI‐69A11 starting at 10 weeks or as late as 24 weeks after the induction of mutant Nras expression inhibited melanoma development (100 and 36%, respectively). BI‐69A11 treatment did not inhibit the development of histiocytic sarcomas, which constitute about 50% of the tumors in this model. BI‐69A11‐resistant Nras^(Q61K)::Ink4a^?/? tumors exhibited increased CD45 expression, reflective of immune cell infiltration and upregulation of gene networks associated with the cytoskeleton, DNA damage response, and small molecule transport. The ability to attenuate the development of NRAS mutant melanomas supports further development of BI‐69A11 for clinical assessment.     

The N-terminal domain of the regulatory subunit is sufficient for complete activation of acetohydroxyacid synthase III from Escherichia coli

We have previously proposed a model for the fold of the N-terminal domain of the small, regulatory subunit (SSU) of acetohydroxyacid synthase isozyme III. The fold is an alpha-beta sandwich with betaalphabetabetaalphabeta topology, structurally homologous to the C-terminal regulatory domain of 3-phosphoglycerate dehydrogenase. We suggested that the N-terminal domains of a pair of SSUs interact in the holoenzyme to form two binding sites for the feedback inhibitor valine in the interface between them. The model was supported by mutational analysis and other evidence. We have now examined the role of the C-terminal portion of the SSU by construction of truncated polypeptides (lacking 35, 48, 80, 95, or 112 amino acid residues from the C terminus) and examining the properties of holoenzymes reconstituted using these constructs. The Delta35, Delta48, and Delta80 constructs all lead to essentially complete activation of the catalytic subunits. The Delta80 construct, corresponding to the putative N-terminal domain, has the highest level of affinity for the catalytic subunits and leads to a reconstituted enzyme with k(cat)/K(M) about twice that of the wild-type enzyme. On the other hand, none of these constructs binds valine or leads to a valine-sensitive enzyme on reconstitution. The enzyme reconstituted with the Delta80 construct does not bind valine, either. The N-terminal portion (about 80 amino acid residues) of the SSU is thus necessary and sufficient for recognition and activation of the catalytic subunits, but the C-terminal half of the SSU is required for valine binding and response. We suggest that the C-terminal region of the SSU contributes to monomer-monomer interactions, and provide additional experimental evidence for this suggestion.     

Acetohydroxyacid synthase from Mycobacterium avium and its inhibition by sulfonylureas and imidazolinones

Tuberculosis (TB) remains one of the world's leading causes of death from infectious disease. It is caused by infection with Mycobacterium tuberculosis or sometimes, particularly in immune-compromised patients, Mycobacterium avium. The aim of this study was to create a tool that could be used in the search for new anti-TB drugs that inhibit branched-chain amino acid (BCAA) biosynthesis, as these are essential amino acids that are not available to a mycobacterium during growth in an infected organism. To this end, we cloned, overexpressed, purified and characterised for the first time an acetohydroxyacid synthase (AHAS), a key enzyme in the pathway to the biosynthesis of the BCAAs, from the genus Mycobacterium. Nine commercial herbicides of the sulfonylurea and imidazolinone classes were tested for their influence on this enzyme. Four of the sulfonylureas were potent inhibitors of the enzyme. The relative potency of the different inhibitors towards the M. avium enzyme was unlike their potency towards other AHASs whose inhibitor profile has been reported, emphasising the advantage of using a mycobacterial enzyme as a tool in the search for new anti-TB drugs.     

Ultrasonographic assessment of bone maturity in newborns

BACKGROUND: Traditionally, the bone maturity at birth has been estimated from the radiological presence and size of the ossified distal femoral epiphysis. This study was conducted in a search for a sonographic tool for the evaluation of neonatal bone maturity. METHODS: We examined sonographically 256 neonates within 24 h of birth. Gestational ages ranged from 36 to 42 weeks (mean: 39.4; median: 40). Birth weights ranged from 1,945 to 5,000 g (mean: 3,175; median: 3,180). The distal femoral epiphysis was imaged on the coronal plane sonogram of the distal femur with the knee at 90 degrees flexion and the distal femoral epiphysis maximal height was recorded. The acetabulum was imaged using Graf's method in the coronal plane image and the acetabular diameter recorded. RESULTS: It was found that plotting the distal femoral epiphysis against neonatal birth weight and gestational age provided a simple method for assessing the bone maturity. According to our study, a neonate can be regarded as bone maturity percentile X when plotting distal femoral epiphysis height or acetabulum diameter against birth weight and gestational age or when averaging the four readings. CONCLUSIONS: We suggest performing sonography of the distal femoral epiphysis as a bedside tool for the assessment of skeletal maturity in newborns.     

Phospholipase C and termination of G-protein-mediated signalling in vivo

In Drosophila photoreceptors, phospholipase C (PLC) and other signalling components form multiprotein structures through the PDZ scaffold protein INAD. Association between PLC and INAD is important for termination of responses to light; the underlying mechanism is, however, unclear. Here we report that the maintenance of large amounts of PLC in the signalling membranes by association with INAD facilitates response termination, and show that PLC functions as a GTPase-activating protein (GAP). The inactivation of the G protein by its target, the PLC, is crucial for reliable production of single-photon responses and for the high temporal and intensity resolution of the response to light.