Table 1

Bacterial strains and plasmids used
Material	Characteristics	References

Strains	Genotype
S. meliloti
GMI211	SmR Nod⁺ Fix⁺	[20]
GMI401	GMI211 asnO::Tn5 SmR NmR	This work
GMI5704	GMI211 fixJ2.3::Tn5 SmR NmR BleoR	[18]
E. coli
ER	asnA31 asnB32 thi	[15]
MM294	Pro-82 thi-1 hsdR17 supE44 endA1	[21]
DH5 α	endA1 hsdR17 (r_k^-mk⁺) supE44 thi-1 recA1 gyrA relA1Δ (lacIZYA-	[29]
	argF)U169 deoR (φ 80dlacΔ (lacZ)M15)

Plasmids

pMF457	pGD926 (IncP broad host range vector) derivative carrying a fixK-
	lacZ fusion with a mutation in the fixT promoter. TcR
pCHK57	pGMI41211 (Inc-P1 broad host range vector) derivative carrying a	[31]
	nifA-lacZ fusion. TcR
pMF11	Same as pMF10 but with fixT in reverse orientation. GmR
pBH1	pBBR1-MCS3 derivative, carrying an HindIII fragment containing	This work
	the asnO gene and the SMb20482 orf.
pBasn2	pBBR1-MCS3 derivative, expressing asnO under the control of the	This work
	pLac promoter
pUC23	pUC18 digested with EcoRI, carrying the S. meliloti EcoRI	This work
	fragment containing the asnO region with the Tn5 insert. Ap^R,
	Km^R.
BAC37	pBeloBAC11 containing a 100 kb S. meliloti genome fragment	[26]
	carrying the asn-like region of pSymB.
pRK602	PRK600::Tn5, Cm^r, Nm-Km^r	[21]

Bergès et al. BMC Microbiology 2001 1:6 doi:10.1186/1471-2180-1-6