TABLE 2.

CN-Cbl utilization phenotype of cysteine substitutions in the BtuB Ton box and effect of the L8P and V10P substitutions^a

btuB alleleb	CN-Cbl concn (nM) for maximal growth of a strain carrying BtuB with the indicated substitutionc
	Alone	L8P	V10P
WT	0.5	>5,000	>5,000
D6C	0.5	5	500
T7C	0.5	0.5	100
L8C	0.5	NAd	>5,000
V9C	0.5	>5,000	500
V10C	0.5	>5,000	NA
T11C	0.5	5	>5,000
A12C	0.5	500	>5,000

^aThe lowest concentrations of CN-Cbl that allows formation of colonies of sizes comparable to those on methionine-supplemented medium are reported as in Table 1. 

^bThe indicated btuB alleles were present in single gene copy at the att_λ locus. Allele designations are as in Table 1. WT, wild type. 

^cEach btuB allele encoded the cysteine substitution indicated in the left column along with either no other amino acid substitution (Alone) or the L8P or V10P substitution. 

^dNA, not applicable.