紫云英根瘤菌菌株107exo基因簇的遗传互补分析

紫云英根瘤菌菌株１０７经Ｔｎ５插入诱变，得到１２株胞外多糖缺陷型变种，以质粒ｐＭＮ２为载体，从其中７株ＥＰＳ－变种内分别构建了７个Ｒ－Ｐｒｉｍｅ质粒（ｅｘｏＲ＇），大部分变种的ＥＰＳ－表型可被ｅｘｏＲ＇互补，恢复野生型表型（ＥＰＳ＋）。互补表明，１２株ＥＰＳ－变种可分为６个不同的互补群，其中５个在遗传上连锁。ｅｘｏＲ＇酶切分析，除ｅｘｏＲ＇－０２，ｅｘｏＲ＇－０４外，其余５只的外源片段均整合于ＰＭＮ２的两同向重复序列ＩＳ２１之间。

Genetic Analysis of a Cluster of exo Genes Required for EPS Synthesis in Rhizobium astragalus

Twelve exopolysaccharide (EPS ) deficient (Exo-) mutants were isolatedafter Rhizobium astragalus strain 107 wassubjected to transposon Tn5 mutagenesis. Using the broad host range vectorplasmid PMN2, a kanamycin-sensitivederivative of plasmid R68. 45, we isolated seven R-primes (exoR' )from seven EPS- mutunts of Rhizobium astragalus strain 107. Each exoR'plasmid carrieda specific Tn5 insertion site and itsflanking fragment coding for exopolysaccharide synthesis (Fig. 1 ).When these exoR' plasmids were introduced back into the twelve mutunts, exoR' plasmids were able to complementthe EPS- phenotype of most mutants(Table 1 ). Based on complementationanalysis, twelve EPS mutunts wereclassified into six distinct complementution groups (Fig. 2). Five of the six exoloci were genetically linked. By restriction enzyme analysis, it was shown thatin all exoR' plasmids but exoR'-02 andexoR'-4, the foreign DNA fragmentswere integrated between the tandem rePeat sequence IS21 (Fig. 1 ). The mobilization of megaplasmid and Southernhybridization experiments showed thatthe probeexactly hybridized the toealDNA of these strains with the pRa107b(Fig. 3). It means that the five linkedexo loci were located on the megaplasmid pRa 1 07b.