The human genome is rich in sequences which are structurally related to the 7SL 
RNA component of the signal recognition particle. The 7SL DNA sequence family 
consists of four 7SL genes, 500 7SL pseudogenes (which are truncated at one or 
both ends of the 7SL sequence) and 500,000 Alu sequences. Both 7SL genes and Alu 
elements are transcribed by RNA polymerase III, and we show here that the 
internal 7SL promoter lies within the Alu-like part of the 7SL gene. Why then 
does RNA polymerase III transcribe the few 7SL genes so efficiently, while 
transcripts from the far more abundant Alu elements are not readily detectable? 
We find that a human 7SL gene and a synthetic Alu sequence derived from it are 
expressed 50-100-fold more efficiently in vitro than either a representative Alu 
element or two 7SL pseudogenes. 5' Deletion and insertion mutants of the 7SL 
gene demonstrate that, in conjunction with the internal promoter, the first 37 
nucleotides upstream from the transcription start site are essential for 
efficient and accurate initiation in vitro. We suggest that the genomic 
sequences upstream from most Alu elements and 7SL pseudogenes do not contain 
this element, and consequently that only a small subset of such sequences can be 
transcribed in vivo. This may help to explain the homogeneity of the Alu family 
within each mammalian genome, as well as the species-specific differences 
between mammalian Alu families.