Gene Regualtion in Animal Cells 
a critical part of the ability of these viral proteins 
to induce oncogenic transformation. 
Gene Regulation by Alternative 
RNA Processing 
Very often a transcription unit encodes not one 
mRNA but several (and thus several proteins) . By 
alternative processing of the primary transcript, 
one particular mRNA is selected and produced. In 
several instances, this is a regulated event, sub- 
ject to change depending on the circumstances of 
the cell. Alternative processing can involve both 
the selective splicing of exons and the selection 
of one of several poly (A) addition sites, thus gen- 
erating a unique mRNA 3' terminus. 
Possibly the best example of regulation of 
poly(A) site utilization is within the differentiat- 
ing B lymphocyte, where there is a dramatic 
change in the nature of the immunoglobulin (Ig) 
heavy-chain polypeptide that is synthesized. Ini- 
tially, a membrane-bound form of the protein is 
produced, but this then switches to a secreted 
form when the cells differentiate to plasma cells. 
The RNAs that direct the synthesis of these two 
proteins are encoded in the same transcription 
unit and are produced by differential cleavage of 
the primary transcript at two different poly(A) 
sites. 
A definition of the events and factors involved 
in poly (A) site formation, again using simple 
viral systems but also the developmentally regu- 
lated Ig heavy-chain gene system, has been a ma- 
jor goal of our laboratory. Experiments have 
identified sequences within the two Ig ^ heavy- 
chain poly(A) sites that are essential for regulated 
selection during B cell differentiation. In addi- 
tion, we have utilized cell-free systems capable of 
accurate poly(A) site processing in vitro to iden- 
tify and isolate factors mediating the processing 
reaction. 
Purification of factors from these cell-free ex- 
tracts has now yielded information concerning 
the multiplicity and nature of factors involved in 
this processing event. At least four distinct activi- 
ties are required for efficient processing at the 
poly (A) site. Two of these factors form com- 
plexes with the pre-mRNA, and the combined in- 
teraction results in the generation of a stable, 
committed complex that allows processing to 
take place. Furthermore, the stability of these 
complexes can vary, depending on sequences in 
the RNA that are known to be important for effi- 
cient processing, indicating that the formation of 
these protein-RNA interactions plays a significant 
role in determining the efficiency with which a 
particular poly(A) site is utilized. 
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