Table II.

The Effects of Mutating Conserved Dra Snf2 Intein Block B and Block F Residues

Residue	Mutation	Temp (°C)	Splice (MP+I)	Precursor (MIP)	N-cleavea (M+IP)	C-cleavea (MI+P)
Block B
Asp61	Ala	37		++++ b
	Ala	15	++			++
	Ser	37	+++	++
	Ser	15	++++
	Thr	15/37	++++
His64	Ala	37		++++
	Ala	15		++		+++
	Asn	37		++++
	Asn	15		++		+++
Trp66	Ala	15/37		++++
	Tyr	37		++++
	Tyr	15	++++
Block F
Cys323	Ala	15/37		+		++++
	Asp	37		++++
	Asp	15				++++
	Thr	37		++++
	Thr	15		++++
	Ser	37		+++		++
	Ser	15				++++
	Gly	15/37		+		++++
	Glu	37		++++
	Glu	15				++++

^aN-cleave and C-cleave = off-pathway single splice junction cleavage reactions. No double cleavage or BI was observed in Table II mutants. Abbreviations as in Figure 3.

^bKey to the relative amount of MIP substrate or products: blank, none detected; +, <10%; ++, 10–50%; +++, 51–90%; ++++, >90%.