Summary By insertional and deletional marker replacement mutagenesis the common
nod region of
Bradyrhizobium japonicum was examined for the presence of additional, essential nodulation genes. An open reading frame located in the 800 bp large intergenic region between
nodD1 and
nodA did not appear to be essential for nodulation of soybean. Furthermore, a strain with a deletion of the
nodI- and
nodJ-like genes downstream of
nodC had a Nod
+ phenotype. A mutant with a 1.7 kb deletion immediately downstream of
nodD1 considerably delayed the onset of nodulation. This region carried a second copy of
nodD (
nodD2). A
nodD1-nodD2 double mutant had a similar phenotype to the
nodD2 mutant. Using a 22-mer oligonucleotide probe partially identical to the
nod box sequence, a total of six hybridizing regions were identified in
B. japonicum genomic DNA and isolated from a cosmid library. Sequencing of the hybridizing regions revealed that at least three of them represented true
nod box sequences whereas the others showed considerable deviations from the consensus sequence. One of the three
nod box sequences was the one known to be associated with
nodA, whereas the other two were located 60 to 70 kb away from
nif cluster I. A deletion of one of these two sequences plus adjacent DNA material mmutant 308) led to a reduced nodulation on
Vigna radiata but not on soybean. Thus, this region is probably involved in the determination of host specificity.Dedicated to Prof. Giorgio Semenza on the occasion of his 60th birthday
相似文献