TRAI, A LUXI HOMOLOG, IS RESPONSIBLE FOR PRODUCTION OF CONJUGATION FACTOR, THE TI PLASMID N-ACYLHOMOSERINE LACTONE AUTOINDUCER

Conjugal transfer of the nopaline-type Agrobacterium Ti plasmid pTiC58 is regulated by a transcriptional activator, TraR, and a diffusible s ignal molecule, conjugation factor (CF). CF is a member of a family of substituted homoserine lactones (HSLs) that act as coinducers for reg ulating gene expression in diverse Gram-negative bacteria by a mechani sm called autoinduction. In Vibrio fischeri HSL production is conferre d by the luxI gene. Homologues of this gene are responsible for HSL pr oduction by other Gram-negative bacteria. A gene that we call tra1, co nferring production of material with CF activity, was localized to a 1 -kb region at the upstream end of tra3 of pTiC58. Spectroscopy showed that the activity was authentic CF. Sequence analysis Showed that tra1 could encode a protein of 211 amino acids, TraI, that is related to t he proteins responsible for HSL production by other bacteria. A second , partial open reading frame immediately downstream of traI could enco de a protein related to TrbB of plasmid RP4, which is required for con jugal transfer. Transcription of tra1 and of the downstream tra3 genes requires TraR and CF and initiates from the traI promoter. The result s show that traI is responsible for CF production, that it is the firs t gene of the tra3 operon, and that expression of this operon is regul ated by autoinduction.