ABSTRACT: Escherichia coli strain MG1655 was grown to mid-log phase in defined aerobic media. One sample was treated with 30 µM CORM-3, the other sample was a control. The volume (30 ml), temperature (37oC) and shaking (200 rpm) were constant. After 15 min of exposure to CO-RM, samples were taken from treated and control cells, harvested into ice cold phenol ethanol (187 µl phenol, 3.56 ml ethanol) to stabilize RNA, and total RNA was purified using Qiagenâs RNeasy Mini kit as recommended by suppliers. RNA was quantified using a BioPhotometer (Eppendorf). Biological experiments were carried out four times, and a dye swap performed for each experiment, providing two technical repeats for each of the four biological repeats. Cells were grown in defined minimal medium (aerobic). Final concentrations are: glycerol (54 mM), K2HPO4 (23 mM), KH2HPO4 (7.3 mM), NH4Cl (18.7 mM), CaCl2 (69 µM), and K2SO4 (15 mM). Trace elements were added at 10 ml per l; final concentrations are: 134 µM EDTA, 31 µM FeCl3, 6.15 µM ZnO, CuCl2 0.57 µM, CoNo3 0.34 µM, 1.6 µM H3BO3, 1 µM ammonium molybdenate and 1 µM sodium selenite. MgCl2 was added at 1 ml per l. All chemicals were of AnalaR grade of purity or higher. E. coli strain MG1655 was grown in 250 ml side arm flasks in 30 ml of the above defined minimal media (aerobic). CORM-3 was added at a final concentration of 30 µM, chosen because it causes only a slight reduction in the growth rate. Cells were grown to mid-log (Klett values of 30-40) before addition of CO-RM. The total 30 ml volume was harvested into ice cold phenol ethanol (187 µl phenol, 3.56 ml ethanol) to stabilize RNA, and total RNA was purified using Qiagenâs RNeasy Mini kit as recommended by suppliers. RNA was quantified using a BioPhotometer (Eppendorf). Biological experiments were carried out four times, and a dye swap performed for each experiment, providing two technical repeats for each of the four biological repeats. Equal quantities of RNA from control and CO-RM-treated cells were labelled by using nucleotide analogues of dCTP containing either Cy3 or Cy5 fluorescent dyes (Perkin Elmer). For each microarray slide, one sample was labelled with Cy3-dCTP, while the other sample was labelled with Cy5-dCTP. RNA (approximately 9 micrograms) was annealed to 4.5 micrograms pd(N)6 random hexamers (Amersham Biosciences) by heating at 65 oC for 10 min, followed by 10 min at 22 oC and cooling on ice for 2 min. This was supplemented with 6 microlitres of 5à First-strand buffer (Invitrogen), 2.5 microlitres dNTP mixture (5 mM each dATP, dTTP, dGTP and 2 mM dCTP), 3 microlitres 0.1 M DTT, 2 microlitres 1 mM Cy3 or Cy5 (Perkin Elmer) and 1.5 microlitre Superscript III reverse transcriptase (200 U) (Invitrogen). This was incubated at 42 oC for 3 hours. The reaction was terminated by the addition of NaOH (7.5 microlitres of 1M) and HCl (7.5 microlitres of 1M) before the addition of TE buffer (300 microlitres). Labelled cDNA was purified using a Qiaquick PCR purification kit (Qiagen). The Cy3-dCTP-labelled sample was mixed with the Cy5-dCTP-labelled sample, vacuum dried and re-suspended in 120 microlitres salt-based hybridisation buffer (supplied with the microarray slides). This was heated at 95 oC for 3 min then cooled on ice for 3 min. The mixture was pipetted onto a microarray slide, sealed with a GeneFrame and coverslip in a hybridization chamber and incubated for 18 h at 42 °C. Following hybridization, microarray slides (minus GeneFrame and coverslip) were washed in a series of pre-warmed (37 °C) SSC buffers for 5 min each at 37oC: 2à SSC/0.1 % SDS, 1à SSC, 0.2à SSC and 0.01à SSC. Microarray slides were dried by centrifugation at 1500 à g for 2 min before scanning. Slides were scanned on an Affymetrix 428 scanner. The average signal intensity and local background correction were obtained using a commercially available software package from Biodiscovery, Inc (Imagene, version 4.0 and GeneSight, version 3.5). Spots automatically flagged as bad, negative or poor in the Imagene software were removed before the statistical analysis was carried out in GeneSight. The mean values from each channel were log2 transformed and normalised using the Lowess method to remove intensity-dependent effects in the log2(ratios) values. The Cy3/Cy5 fluorescent ratios were calculated from the normalized values. Biological experiments (i.e. a comparison of control and plus CO-RM cells) were carried out four times, and a dye swap performed for each experiment, providing two technical repeats for each of the four biological repeats. Data from the independent experiments were combined. Genes that were differentially expressed ⥠twofold and displayed and P value of < 0.05 (as determined by a t test) were defined as being statistically significantly differentially transcribed.
