The plasmid SCP2, initially discovered through the occurrence of a high fertility variant, SCP2*, is a self-transmissible fertility factor capable of promoting chromosomal recombination within A3(2). Further high fertility variants of SCP2, similar to SCP2*, were isolated from amongst the recombinants produced in matings involving SCP2, and their genetic properties were compared. SCP2 and its derivatives elicit lethal zygosis on transfer into an SCP2 recipient; this plasmid-determined phenotype allowed the isolation of SCP2 strains and the detection of the interspecific transfer of SCP2* by mating from to and , whereupon it underwent stable maintenance. The transfer genes of SCP2 and SCP2*, which are not normally fully expressed, were shown to undergo transient derepression on entry into an SCP2 strain. An ‘entry disadvantage’ system determined by SCP2 and SCP2* was recognized.


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