There are two well-documented trans-acting factors required for histone pre-mRNA processing. These are:
1 Stem-loop binding protein (SLBP), also termed hairpin binding protein (HBP). This 32 kDa protein is likely the first protein that binds to the histone pre-mRNA as it is being transcribed.
The U7 snRNP. This particle contains the U7 snRNA, the smallest of the snRNAs which varies from 57-70 nts long depending on the species. The 5' end of U7 snRNA binds to a sequence 3' of the stemloop, termed the histone downstream element (HDE). There are a number of proteins found in the U7 snRNP. There are 7 Sm proteins, as are present in the spliceosomal snRNP. Five of these proteins are the same as ones found in the spliceosomal snRNPs and there are 2, Lsm10 and Lsm11 that are unique to U7 snRNP.
A third protein joins the U7 snRNP, ZFP100, a large zinc finger protein. ZFP100 interacts with SLBP bound to the histone pre-mRNA and with Lsm11 and likely plays a critical role in recruiting U7 snRNP to the histone pre-mRNA.
It should be noted that there must be other trans-acting factors, including the factor that catalyzes the cleavage reaction. The cleavage occurs in the presence of EDTA as does the cleavage reaction in polyadenylation, it is likely that this reaction is catalyzed by a protein. There may well be additional proteins associated with U7 snRNP, and since under some conditions in vitro processing occurs in the absence of SLBP, it is possible that all of the other factors required for processing are associated with the active form of U7 snRNP.