In yeast, a new mutation called suc was recently discovered.  The suc mutant cannot use sucrose as a source of carbon, and will not grow if the culture medium contains only sucrose as a carbon source.  A plasmid library made with cDNA prepared from wild type yeast was used to transform a strain of yeast with two mutations: suc and ura.  In this strain the URA gene, which allows yeast to grow in medium depleted in uracil, has been deleted. After transformation the yeast cells were plated on culture medium without uracil and containing sucrose as the only carbon source.
 
a)  Why must the plasmid chosen to construct the library contain a functional URA gene?
 
b)  Although 80% of the yeast were transformed, only a fraction of the transformed yeast
formed colonies on the medium.  Could this experiment be called a complementation
assay?  Explain your reasoning.
 
c)  The recombinant plasmid extracted from the surviving colonies contains a gene called
SUC2 that codes for convertase, an enzyme needed to metabolize sucrose.  Mutations in
the SNF gene also lead to an inability to ferment sucrose, but SNF mutants can still
ferment glucose.  Although the SNF mutants have no mutations in the SUC2 gene, they
have no convertase activity.  We know that the SNF gene is not involved in translation. 
Is the SNF gene involved in DNA replication or transcription?  Briefly justify your
answer.
 
d)  The SNF mutation is rescued by another mutation in the gene for histone H4.  What is
the likely function of the SNF gene?