Abstract: Atransposon Tn5 insertion mutant ON-2 of Rhizobium, fredii wild strain QB1130 was used in ecological study to evaluate the probability of horrizontal transfer of transposon Tn5 in natural environments and its influence on the movement of mutants. The results show that the horrizontal transfer frenquency of Tn5 is very low in natural moist soil. The Km^tphenotype of the mutant associated with Tn5 insertion is shown to be stable after growth for over 40 generations on nonselective media. No significant difference(P=0.01) between the movements of the mutant and its corresponding wild type was found, which implies that transponson Tn5 does not influence the movement of Tn5 mutant. The study suggests that Tn5 can be selected as a stable ecological marker in monitoring R. fredii genetic engineering strains used in the field.

Key words: Transposon Tn5, Ecological marker, Soil water potential, Gene horrizontal transfer