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Appl Environ Microbiol, February 1998, p. 419-426, Vol. 64, No. 2
Department of Biology, University of York,
York YO1 5YW, United Kingdom,1 and
Department of Applied Chemistry and Microbiology, Biocenter 1A,
FIN-00014 University of Helsinki, Helsinki, Finland2
Received 7 July 1997/Accepted 5 October 1997
The diversity and phylogeny of nodA and
nifH genes were studied by using 52 rhizobial isolates from
Acacia senegal, Prosopis chilensis, and related
leguminous trees growing in Africa and Latin America. All of the
strains had similar host ranges and belonged to the genera
Sinorhizobium and Mesorhizobium, as previously determined by 16S rRNA gene sequence analysis. The restriction patterns
and a sequence analysis of the nodA and nifH
genes divided the strains into the following three distinct groups:
sinorhizobia from Africa, sinorhizobia from Latin America, and
mesorhizobia from both regions. In a phylogenetic tree also containing
previously published sequences, the nodA genes of our
rhizobia formed a branch of their own, but within the branch no
correlation between symbiotic genes and host trees was apparent. Within
the large group of African sinorhizobia, similar symbiotic gene types
were found in different chromosomal backgrounds, suggesting that
transfer of symbiotic genes has occurred across species boundaries.
Most strains had plasmids, and the presence of plasmid-borne
nifH was demonstrated by hybridization for some examples.
The nodA and nifH genes of Sinorhizobium
teranga ORS1009T grouped with the nodA
and nifH genes of the other African sinorhizobia, but
Sinorhizobium saheli ORS609T had a totally
different nodA sequence, although it was closely related
based on the 16S rRNA gene and nifH data. This might be because this S. saheli strain was originally isolated from
Sesbania sp., which belongs to a different cross-nodulation
group than Acacia and Prosopis spp. The factors
that appear to have influenced the evolution of rhizobial symbiotic
genes vary in importance at different taxonomic levels.
0099-2240/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Three Phylogenetic Groups of nodA and nifH
Genes in Sinorhizobium and Mesorhizobium Isolates
from Leguminous Trees Growing in Africa and Latin America
*
Corresponding author. Present address: Department of
Applied Chemistry and Microbiology, Division of Microbiology, P.O. Box 56, Biocenter 1A, FIN-00014 University of Helsinki, Helsinki, Finland.
Phone: 358-9-708-59202. Fax: 358-9-708 59322. E-mail: Kaisa.Haukka{at}Helsinki.Fi.
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