PolyA+ purification with oligodT cellulose

[srcksac at chv.lincoln.cri.nz]

Dear netters - I am purifying RNA from onion tissues.  After CsCl pelleting I 
get RNA with a 260/280 of ca. 1.7-1.8.  I then extract the polyA RNA using the 
Pharmacia Micro mRNA purification Kit, or using my own solutions, and oligodT 
cellulose.  I have carried out this method successfully with a variety of 
tissues and got mRNA with 260/280's of >2.0.  However a recent batch of RNA, 
which after CsCl pelleting gave a 260/280 of ca.1.7, gave a 260/280 after 
polyA extraction of 1.4.  Does anyone know what would cause this? Is it Cs 
contamination, and if so can the Cs be removed by eg. G50 spin columns?  Also 
would anyone like to recommend their favourite recipes for oligodT-cellulose 
polyA extractions (binding, wash and elution buffers)?  Many thanks in advance 
for help with this.

Cheers
Sonya Clark
Crop & Food Research
Lincoln
New Zealand
E-mail srcksac at chv.lincoln.cri.nz