I certify that I have read this study and in my opinion 

 it conforms to acceptable standards of scholarly presenta- 

 tion and is fully adequate, in scope and quality, as a dis- 

 sertation for the degree of Doctor of Philosophy. 



)c*i-*' Cvv-xx-iWo-. .-.^ ^ . r^vv 



%■ 



Keelnatham T. Shanmugam 



Professor of Microbiology and Cell Science 



I certify that I have read this study and in my opinion 

 it conforms to acceptable standards of scholarly presenta- 

 tion and is fully adequate, in scope and quality, as a dis- 

 sertation for the d^g-cee of Doctor ,,4x£ Philosophy. 



Richard P. Boyce 

 Professor of Biochemistry and Molecular 

 Biology 



This dissertation was submitted to the Graduate Faculty 

 of the College of Agriculture and to the Graduate School and 

 was accepted as partial fulfillment of the requirements for 

 the degree of Doctor of Philiosophy. 



April 1994 



Dean, 



'a^A /. t^ 



^ 



lege of Agriculture 



Dean, Graduate School 



