in the highly reproducible pattern of puffs that 
arise in the polytene chromosomes. Much of Dr. 
Thummel's research is focused on characterizing 
the temporal control of E74 expression, in an effort 
to understand how the timing of gene expression is 
regulated during development. The timing of E74A 
expression appears to be controlled at both tran- 
»w scriptional and translational levels. 
Ecdysone induces synthesis of the E74A mRNA at 
the level of transcriptional initiation, in a manner 
similar to glucocorticoids. RNA polymerase moves 
along the E74 gene at a rate of 1.1 ± 0.3 kb/min, in 
agreement with the in vivo transcription rate de- 
scribed by others. Furthermore, as expected from 
the length of the gene, cytoplasmic E74A mRNA can 
first be detected 60 min after the addition of ecdy- 
sone. This supports a model that gene length in 
Drosophila may provide a way to control the time 
between the inducer signal (ecdysone) and the 
product {E74A mRNA). The observation that E74A 
mRNA appears in the cytoplasm shortly after tran- 
scription termination suggests that processing of 
the primary transcript is not rate limiting. This has 
been confirmed by RNase protection assays, which 
reveal splicing of nascent E74A transcripts prior to 
polyadenylation. 
PUBLICATIONS 
The appearance of E74A protein may be fur- 
ther delayed at the post-transcriptional level. The 
E74A mRNA contains an unusually long 5' leader, 
1,891 nucleotides in length, with 11 short open 
reading frames. This leader may function to regu- 
late translation of the E74A transcript. Initial 
Western blotting experiments indicate that E74A 
protein is present at high levels at the white 
prepupal stage, several hours after the mRNA be- 
gins to accumulate to high levels. This delay in 
expression may help confer tissue specificity to 
the expression of E74A. High levels of E74A 
mRNA can be detected by in situ hybridization in 
virtually all late larval tissues. By the white pre- 
pupal stage, however, the RNA distribution is more 
restricted, with high levels in the central nervous 
system and imaginal disks. It will be interesting to 
see if this pattern correlates with the pattern of 
E74A protein accumulation. Mutational analysis of 
the 5' leader should indicate whether these se- 
quences exert a regulatory role in E74A protein 
synthesis. 
Dr. Thummel is also Assistant Professor of 
Human Genetics at the University of Utah School of 
Medicine. 
Articles 
Thummel, C.S. 1989. The Drosophila E74 promoter contains essential sequences downstream from the start 
site of transcription. Genes Dev 3:782-792. 
Thummel, C.S., Boulet, A.M., and Lipshitz, H. 1988. Vectors tor Drosophila P-element-mediated transforma- 
tion and tissue culture transfection. Gene 74:445-456. 
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