T w o methods were employed to test whether Synechococcus
WH 7803 produced siderophores when grown under iron-limited c o n d itions.
Initially t h e universal chemical assay described by Schwyn 6
Neilands (1987) was used. However, the EDDA bioassay 64
described by Kerry et al. (1988) proved more appropriate. All cultures were grown for a period of 5-7 days in chelator- deficient medium to ensure that cellular iron levels were depleted, prior to inoculation into normal medium containing EDDA concentrations ranging from 0-75 nq ml- 1 . Cultures were grown under the same conditions as described in Maintenance of Cultures. Growth was determined at OD7 5 0 nm-
2.20 Localization studies for the 36 kDa iron stress protein
2.20.1 Periplasmic fraction
50 ml samples of Synechococcus WH 7803 were treated for isolation of the periplasmic fraction in three ways:- i) osmotic shock as described by Neu & Heppel (1965). ii) chloroform extraction described as by Ames et al. (1984). iii) osmotic shock described by Thiel (1988).
The osmotic shock method of Neu & Heppel (1965) w a s as follows. To a bacterial culture, one-tenth volume of 0.5 M Tris-HCl, pH 7.8 was added and incubated for 10 min at room temperature. Harvested cells were resuspended in 800 Ml sucrose solution (30 mM Tris-HCl pH 7.8, 40% [w/v] sucrose, 2 a M e d t a ), incubated for 10 min in 1.5 ml Eppendorf tubes at room temperature. Cells w ere pelleted by centrifugation for 5 min in an Eppendorf centrifuge, the supernatant was discarded and the cells resuspended in 500 Mi of ice-cold distilled water and incubated on ice for 10 min. Cells were pelleted by centrifugation for 5 min in an Eppendorf centrifuge, the supernatant containing the periplasmic fraction was removed
and stored at -20*C. The remaining cells were resuspended in buffer (10 mM Hepes pH 7.2, 10 mM M g C l 2 ), and treated with Triton-X-100 (2% [v/v] final concentration) for 30 min at room temperature, PMSF, a protease inhibitor, ( 1 mM final
concentration) was added to this suspension. Whole cells were removed from the Triton-X-100 soluble cell lysate by
centrifugation in an Eppendorf centrifuge for 5 min. The cell lysate fraction was stored at -20*C. Remaining Eschorichia coli cells used for control purposes were resuspended in 500
Hi 30 mM Tris-HCl pH 7.8 and subjected to two 15 sec bursts at an amplitude of 18 n in a Jencons sonicator, with a 15 sec cooling time in between pulses. During sonication cells were kept on ice. The cytoplasmic fraction was removed from the cell debris by centrifugation in an Eppendorf centrifuge and stored at - 2 0 *C.
A chloroform extraction procedure was also used to obtain periplasmic proteins. Harvested cells (20 ml) were
resuspended in residual medium by brief vortexing, 2 0 fil of chloroform was added, after brief vortexing the suspension was incubated at room temperature for 15 min. Buffer 0.2 ml, (0.01 M Tris-HCl pH 8.0) was added, a n d the cells were removed from the periplasmic fraction by centrifugation in an Eppendorf centrifuge for 1 min. The supernatant was carefully withdrawn using a Pasteur pipette and stored at -2 0*C.
The method described by Thiel (1988) was also used to obtain periplasmic proteins. Harvested cells were resuspended in twice the original volume of buffer (10 mM TES, 0 . 2 M MgCl2
pH 7.3,) and incubated at 25*C for 20 min. The cells were centrifuged at 3,000 rpm in a Gallenkamp centrifuge for 10 min and resuspended rapidly in half the original volume of distilled water, the suspension was maintained at 25*C for a further 2 0 min. The supernatant obtained after centrifugation as described above was stored at -20*C. The remaining cells were treated with Triton-X-100 as described above.
2.20.1.1 Alkaline phosphatase assay
The method of Brickman and Beckwith (1975) was used. 100 Ml of a periplasmic sample was added to 900 )il of 1 H Tris-HCl pH 8.0 in a microfuge tube. 100 Ml of either ASW or SOB medium w a s used as an appropriate blank. 1 0 0 Ml of freshly prepared 4 mg ml- 1 Sigma 104 in 1 M Tris-HCl pH 8 was added and the tim e recorded. All tubes were incubated at 37*C until a faint y e l l o w colour appeared, the reaction was stopped by the addition of 100 Ml of 1 M K2HP04 (filter sterile) and the time recorded.
The absorbance at 420 nm was determined and the data was expressed as the change in OD 420 min" 1 ml" 1 mg"1 .
2.2 0 .2
C e l l f r a c t i o n a t i o n p r o c e d u r e2.20.2.1 Cell breakage
Unless stated otherwise samples were kept on ice during this procedure.
Radioactively labelled [3 5S-methionine] cells were used as a marker for the 36 kDa low iron induced protein. A culture (100 ml) of Synechococcus WH 7803 labelled with 3 5S- methionine, as described previously was harvested and resuspended in 1 ml of Tris-HCl buffer (20 mM Tris-HCl pH 8), giving a final concentration of 0.2 g cells ml- 1 . To this cell suspension 3 ml of a 0.2 g ml- 1 suspension of unlabelled cells, (harvested from a 1 0 litre volume and grown as described in Section 2.6) were added such that the radioactivity was dil u t e d to one-quarter of the original value. A 200 nl sample was removed for control purposes. The remaining cells were homogenized using a Braun homogenizer (Section 2.15) using a bead size of 0.1 mm - 0 . 1 1 mm for 15 min under constant cooling. Up to 2 g of cells could be homogenized per tube, however, larger cell masses required longer times for homogenization. The homogenate was washed from the beads using 2 0 m M Tris-HCl pH 8 buffer and PMSF was added to a final concentration of 1 mM. Cell debris and unbroken cells were removed from the homogenate by centrifugation at 4,500 rpm for 15 min in a Gallenkamp centrifuge. Any unbroken cells were resuspended in 20 mM Tris-HCl buffer and rehomogenized in order to achieve a maximum yield of cell material.
2.20.2.2 Differential centrifugation and detergent extraction
The membrane fraction was removed from the homogenate by ultracentrifugation at 35,000 rpm for 60 min at 4*C using a Beckman SW40Ti rotor in a Beckman L8 ultracentrifuge. The supernatant, (the cytoplasmic fraction - purple-pink in colour) was removed and stored at -2 0*C. The membrane pellet, after washing to remove any remaining cytoplasmic fraction, was resuspended by hand homogenization in an equal volume of Tris-magnesium buffer (20 mM Tris-HCl pH 8.0, 10 mM MgCl2 ) and 4% [v/v] Triton-X-100 (in 2 0 mM Tris-HCl pH 8.0, 10 mM MgCl2 ). The internal membrane fraction was removed from the cell wall component by extraction in 2% [v/v] Triton-X- 1 0 0 Tris- magnesium buffer, at 25*C for 60 m i n with constant stirring. The Triton-insoluble cell wall fraction was removed from the internal membrane fraction by ultracentrifugation at 35,000 rpm for 60 min at 4*C using a Beckman SW40Ti rotor in a Beckman L8 ultracentrifuge. The supernatant (internal membrane fraction) was decanted, and the cell wall fraction was resuspended in 500 /il of 20 mM Tris-magnesium buffer. Both samples were stored at - 2 0 *C.
2.20.2.3 Sucrose density centrifugation
Alternatively, the membrane fraction after detergent extraction was layered onto the top of a sucrose gradient consisting of:- 85% [w/v], 80% [w/v], 70% [w/v], 60% [w/v] and 50% [w/v] sucrose in 10 mM Hopes pH 7.2, 10 mM M gCl2 . Polycarbonate centrifuge tubes (17 ml) were used in a Beckman
SW28 rotor. The samples were centrifuged at 23,500 rpm for 18 hr at 4 *C in a L8 Beckman ultracentrifuge. Cell walls banding in the 70%-80% [w/v] sucrose interface were r emoved from the gradient by piercing the side of the tube with a syringe and allowing the fraction to flow out under positive pressure. The internal membrane fraction was removed from t h e top of the 50% [w/v] sucrose fraction.
2.21 Purification of the 36 kDa protein
The 36 kDa protein was purified from the internal membrane fraction by means of a Fast Protein Liquid Chromatography system (Pharmacia) using an anion exchange c o lumn (Nono Q HR 5/5; Hiload Q-Sepharose 16/20). The type of column used was dependent upon the total protein concentration of the sample.
The internal membrane fraction was concentrated to a volume of 5 ml containing approximately 20 mg of protein (determined by Biorad protein assay) prior to loading onto the M o n o Q anion exchange column (bed volume of 1 ml). The sample was injected via a sample loop o nto the top of t h e column w h i c h had been equilibrated in 10 mM Tris-HCl pH 8.0, 0.1% [v/v] Triton-X-100). Protein was eluted from the column with a linear KC1 gradient from 0-1M over a period of 20 min. Samples with a total protein content of over approximately 29 mg in 10 ml were loaded onto a Hiload Q anion exchange column (bed volume 50 ml) as described above. The larger column size ensured a higher loading capacity. Protein was eluted from the column in the same way as described above with the same
buffers, except that a flow rate of 5 ml min" 1 was used. The salt gradient, was formed between 50 ml-100 ml of elution buffer.
All samples were and stored at 4*C prior to concentration using a stirred cell ultrafiltration unit (Amicon) and SDS- PAGE analysis.
2.22 Amicon concentration of experimental samples Internal membrane samples and fractionated samples from the FPLC purification process were concentrated using a stirred cell ultrafiltration unit (Amicon) with a
nitrocellulose filter having a 1 0 , 0 0 0 molecular weight cut off (Flowgen). Filtration was conducted under pressure from an attached nitrogen source, with rapid stirring.
2.23 Preparation of 36 kOa protein antigen for antibody production
2.23.1 Location of the protein antigen after e 1ectrophor es i s
This was achieved using a non-fixing Coomassie Brilliant blue stain as described by Harlow and Lane (1988). The sensitivity of staining was approximately 1-2 M9/band. After electrophoresis the g e l was washed with three changes in deionized water, then stained for 1 0 min at room temperature in 0.05% [w/v] Coomassie Brilliant blue R prepared in 500 ml water. The gel was destained by washing in several changes of deionized water for 2 hr. The band of interest was excised
from the gel and stored at -20*C until electroeluted from the gel.
2.23.2 Electroelution of the protein antigen
The protein was removed from the gel by electroelution as follows:-
Dialysis tubing, (18/32 inch in size) prepared by boiling in 10 mM EDTA, was rinsed thoroughly in distiled water prior to use. The protein band of interest was transferred to the dialysis tubing containing 5ml SDS-PAGE running buffer. The tubing was placed horizontally across an electrophoresis chamber containing SDS-PAGE running buffer, and the protein eluted at 70 mA for 3 hr. A reverse pulse for 2 0 sec after elution moved the protein away from the tubing edge. The eluant was stored at -20*C in a sterile Universal. Prior to use as an antigen for antibody production the sample was concentrated as described in section 2.2 2.