Molecular cloning and characterization of a genetic region from Serratia marcescens involved in DNA repair

We report here the molecular isolation of a DNA fragment which encodes Tag-like activity from the Gram-negative bacterium Serratia marcescens. A recombinant plasmid encoding Tag-like activity was isolated from a S. marcescens plasmid gene library by complementation of an Escherichia coli tag mutant, which is deficient in 3-methyladenine DNA glycosylase I. The clone complements E. coli tag, recA, alkA, but not alkB, mutants for resistance to the DNA-damaging agent methyl methanesulphonate (MMS). The coding region of the Tag activity, initially isolated on a 6.5kb BamHI fragment, was defined to a 1.8kb BglII-SmaI fragment. Labelling of plasmid-encoded proteins using maxicells revealed that the 1.8kb fragment encodes two proteins of molecular weights 42,000 and 16,000. Data presented here suggest that the cloned fragment encodes a DNA repair protein(s) that has similar activity to the 3-methyladenine DNA glycosylase I of E. coli.     

Submicromolar Ca2+ regulates phosphorylating respiration by normal rat liver and AS-30D hepatoma mitochondria by different mechanisms

The stimulation of 2-oxoglutarate and NAD(+)-isocitrate dehydrogenase by Ca2+ in mitochondria from normal tissues has been proposed to mediate partially the activation of oxidative energy metabolism elicited by physiological elevations in cytosolic Ca2+. This mode of regulation may also occur in tumor cells in which several aspects of mitochondrial metabolism are known to be altered. This study provides a comparison of the stimulation by submicromolar concentrations of Ca2+ on the rates of ATP-generating (state 3) respiration under physiologically realistic conditions by mitochondria isolated from normal rat liver and from highly malignant rat AS-30D ascites hepatoma cells. The K0.5 for activation of glutamate-dependent state 3 respiration by Ca2+ in the presence of ATP at 37 degrees C was determined to be 0.70 +/- 0.05 (S.E.) microM for hepatoma mitochondria and 0.90 +/- 0.03 microM for rat liver mitochondria. This activation was also reflected by a Ca2(+)-induced shift in the oxidation-reduction state of hepatoma mitochondrial pyridine nucleotides to a more reduced level and Ca2+ stimulation of 14CO2 production from [1-14C]glutamate. Whereas the Ca2+ sensitivity of state 3 respiration by hepatoma mitochondria can be explained by the activation of 2-oxoglutarate and possibly NAD(+)-isocitrate dehydrogenases, the Ca2+ sensitivity of liver mitochondrial respiration appears to be predominantly mediated by activation of electron flow through ubiquinone and Complex III of the electron transport chain, as indicated by the specificity of the effects of Ca2+ on respiration with different oxidizable substrates. Although rat liver and hepatoma mitochondria employ different modes of Ca2(+)-activated ATP generation, these results support the hypothesis that changes in cytosolic Ca2+ play a significant role in the potentiation of energy production in tumor, as well as normal tissue.     

Evidence for unique DNA repair activity encoded by a cloned Serratia marcescens gene: suppression of Escherichia coli mutations that reduce repair of alkylated DNA.

A recombinant plasmid containing a Serratia marcescens DNA repair gene has been analyzed biochemically and genetically in Escherichia coli mutants deficient for repair of alkylated DNA. The cloned gene suppressed sensitivity to methyl methanesulfonate of an E. coli strain deficient in 3-methyladenine DNA glycosylases I and II (i.e., E. coli tag alkA) and two different E. coli recA mutants. Attempts to suppress the methyl methanesulfonate sensitivity of the E. coli recA mutant by using the cloned E. coli tag and alkA genes were not successful. Southern blot analysis did not reveal any homology between the S. marcescens gene and various known E. coli DNA repair genes. Biochemical analysis with the S. marcescens gene showed that the encoded DNA repair protein liberated 3-methyladenine from alkylated DNA, indicating that the DNA repair molecular is an S. marcescens 3-methyladenine DNA glycosylase. The ability to suppress both types of E. coli DNA repair mutations, however, suggests that the S. marcescens gene is a unique bacterial DNA repair gene.     

Acetoacetyl-acyl carrier protein synthase. A target for the antibiotic thiolactomycin

The biochemical basis for the inhibition of fatty acid biosynthesis in Escherichia coli by the antibiotic thiolactomycin was investigated. A biochemical assay was developed to measure acetoacetyl-acyl carrier protein (ACP) synthase activity, a recently discovered third condensing enzyme from E. coli (Jackowski, S., and Rock, C.O. (1987) J. Biol. Chem. 262, 7927-7931). In contrast to the other two condensing enzymes in E. coli, acetoacetyl-ACP synthase (synthase III) condensed malonyl-ACP with acetyl-CoA, rather than with acetyl-ACP. The concentration dependence of thiolactomycin inhibition of fatty acid biosynthesis in vivo was the same as the inhibition of acetoacetyl-ACP synthase activity in vitro indicating that the two phenomena were related. A thiolactomycin-resistant mutant (strain CDM5) was isolated. The specific activity of acetoacetyl-ACP synthase in extracts from this mutant was 10-fold lower than in extracts from its thiolactomycin-sensitive parent resulting in a marked defect in the ability of strain CDM5 to incorporate acetyl-CoA into fatty acids in vitro. The residual acetoacetyl-ACP synthase activity in the resistant strain was refractory to thiolactomycin inhibition. In addition, acetyl-CoA:ACP transacylase activity in strain CDM5 was resistant to inactivation by thiolactomycin suggesting that the acetoacetyl-ACP synthase also catalyzes this transacylation reaction. These data point to acetoacetyl-ACP synthase as a target for thiolactomycin inhibition of bacterial fatty acid biosynthesis.     

Optical verification of a technique for in situ ultrasonic measurement of articular cartilage thickness

Several investigators have used pulse-echo ultrasonics to measure the thickness of articular cartilage in situ. The underlying assumption in all measurements was that the second reflection used in thickness calculations was from the calcified-cartilage/cartilage boundary (tidemark). To investigate this assumption, the thickness of 24 cartilage plugs excised from a human femoral head was measured both ultrasonically and optically. Measurements established that the second reflection was from the tidemark and validated the ultrasonic technique as a method of mapping the thickness distribution of articular cartilage in synovial joints in situ.     

Acidosis, Acetazolamide, and Amiloride: Effects on 22Na Transfer Across the Blood-Brain and Blood-CSF Barriers

Sprague-Dawley rats were given treatments, known to decrease 22Na movement into choroid plexus and CSF, to investigate their effect on 22Na transfer across the cerebral capillaries. Acidic salts, acetazolamide, or amiloride was injected intraperitoneally into bilaterally nephrectomized rats, and the rate of 22Na uptake into parietal cortex, pons-medulla, and CSF was determined at 12, 18, and 24 min. Severe acidosis (arterial pH 7.2), produced by HCl injection, decreased the rate of 22Na entry into both brain regions and CSF by 25%, whereas mild acidosis (pH 7.3) from NH4Cl injection reduced brain entry by 18%, but CSF entry by only 10%. Like HCl acidosis, amiloride reduced transport into both brain and CSF by 22%. Penetration of 22Na into parietal cortex was unchanged by acetazolamide, but that into CSF was slowed 30%. Since uptake of 22Na into cortical regions is primarily movement of tracer across the cerebral capillaries when tracer uptake time is less than 30 min, the results indicate that both metabolic acidosis and amiloride decrease Na+ permeativity at the cerebral capillaries as well as at the choroid plexus. Acetazolamide, on the other hand, alters Na+ movement only across the choroidal epithelium.