centrifuged at 10,000 rpm for 10 min, washed with 70% ethanol and dried. DNA was resuspended in 500 J..Ll of TE containing 20 J..Lg/ml RNAase, incubated at RT for 30 min and analyzed by agarose gel electrophoresis. DNA for transfection was prepared using the Plasmid midi kit DNA purification system using protocols described in the manual.

A 1000 ml culture of cells harboring the plasmid were grown 0/N in LB Amp· Next morning the culture was chilled and cells pelleted at 4,500 rpm in a Sorvall SS34 rotor for 20 min. The supernatant was discarded and cells were washed with 100 ml of STE buffer (0.1 M NaCI, 10 mM Tris HCl and 1 mM EDT A, pH 8.0). The pellet obtained after centrifugation was resuspended in I 0 ml of GTE solution containing I mg/ml lysozyme and the mixture was incubated at RT for 20 min at 4oc. Alkaline SDS (20 ml) was added and the mixture was incubated at RT for 10 min after mixing gently by inverting the tube. Ice cold potassium acetate solution ( 15 ml) was added and the tube was chilled on ice for 15 min and then centrifuged at 18,000 rpm at 40C in a SS34 rotor. The supernatant was carefully transferred to a fresh tube, DNA was precipitated by adding 0.6 volume isopropanol and incubating at RT for 10 min and then recovered by centrifugation at 5000 rpm at RT for 30 min. DNA was rinsed with 70% ethanol, dried and dissolved in 3 ml of TE. To the nucleic acid solution 3 ml of chilled LiCI (5 M) was added, mixed and the precipitate removed after spinning at 10,000 rpm for 10 min at 40 C. DNA was precipitated from the supernatant using an equal volume of isopropanol,

The Luria Bertani (LB; pH 7.5) medium was prepared in double distilled water by adding, NaCl 1%, Yeast extract 0.5%, and Tryptone 1% and sterilized by autoclaving under pressure (15 lbslinch2) for 20 min. Solid growth medium was prepared by adding 1.5% agar to LB prior to autoclaving. Appropriate antibiotics were added after cooling the medium to approximately 50-60°C. Bacterial cultures were grown in LB medium at 37°C in an orbital shaker set at 200 revolutions per minute (rpm).

MeancellVolume(MCV).Itisexpressedinfentolitres(1fentolitreorflisequivalentto10'151)andcalculatedby thefollowingformula:PCVMCV=.....................x10(fl)RBC8.10.6.2.MCHMeancellhaemoglobin(MCH)=AverageweightofHbinanerythrocyte.Itisexpressedinpicograms(pg)whichisequivalentto10"12g.Itiscalculatedbythefollowingformula:HbMCH=-----------------x10(ppg)RBC

For estimation of tannase activity the reaction mixture (4 ml) contained 1.0 ml of 1.0% tannic acid (prepared in citrate-phosphate buffer, pH 5.0), 2.0 ml of citrate-phosphate buffer (pH 5.0) and 1.0 ml of appropriately diluted culture supernatant. The reaction mixture was incubated at 40°C for 30 min in a water bath. The reaction was stopped by adding 4.0 ml of 2.0% BSA solution. In the control reaction, BSA was added prior to incubation. Now the tubes were left for 20 min,at room temperature, for precipitating the residual tannins and subsequently centrifuged at 10,000 rpm for 20 min. The end product, gallic acid thus formed was estimated by diluting 20 μl of the supernatant to 10 ml with DDW. Now, the absorbance at 260 nm was read against a blank (DDW) in a UV spectrophotometer (1601, Shimadzu Corporation, Japan). One unit of tannase: One tannase unit is defined as the amount of enzyme that releases 1 μmol of gallic acid from the substrate (tannic acid) per ml per min under standard assay conditions

white colonies were recovered and purified to give growth. If the mutation caused synthetic lethality then white colonies (that lack the shelter plasmid) would not be observed since plasmid loss would result in growth arrest. Therefore, lethality was inferred when either white colonies were not recoveredor were recovered but failed to purify further

To determine whether a particular mutation conferred lethality in the ppGpp0or ΔdksAbackground, an assay was devised based on the use of an unstable, easy to cure shelter plasmidpRC7, similar to that described previously(Bernhardt & de Boer, 2004). In the wild-type strain carrying pRC7, this plasmid can be lost at a frequency of 20-30% in the absence of the selection. However, this will not be seen if the plasmid loss leads to cell death. Since the plasmid pRC7 confers a lac+phenotype, in the absence of the selection plasmid loss can be visualized on X-gal IPTG containing plates as white colonies in a Δlac strain whereas the colonies that retain the plasmid will appear blue.In order to carry outsynthetic lethal screen in the ppGpp0or ΔdksAstrains, the spoT or dksAgenes cloned in pRC7 under the control of lacpromoter were used. Theseshelter plasmids,namely,pRCspoT or pRCdksA, respectivelywere transformed into the ppGpp0or ΔdksAstrain. To test the synthetic growth phenotypes, the mutations of the genes to be tested were introduced by phageP1 transductions. The resultingstrains were grown overnight in LBcontaining the antibiotic selection for the shelter plasmid and IPTG for expression of spoTor dksA, subsequently washedin minimal A medium and dilutions(usually 10−5or 10−6) of these cultureswere spreadon X-gal and IPTG containing plates without antibiotic selection for the shelter plasmid. The phenotypes of the white colonies in comparison with the blue colonies were noted. Viability of the strains was inferred when