THE RHODOBACTER-SPHAEROIDES-2.4.1 RHO-GENE - EXPRESSION AND GENETIC-ANALYSIS OF STRUCTURE AND FUNCTION

The gene which encodes transcription termination factor Rho from Rhodo bacter sphaeroides 2.4.1, the gram-negative facultative photosynthetic bacterium, has been cloned and sequenced, The deduced protein shows a high level of sequence similarity to other bacterial Rho factors, esp ecially those from proteobacteria, However, several amino acid substit utions in the conserved ATP-binding site have been identified, When ex pressed in Escherichia coli, the R. sphaeroides rho gene relieves Rho- dependent polarity of the ap operon, indicating interference with the transcription termination machinery of E. coli. A truncated version of R. sphaeroides Rho (Rho') is toxic to a bacterium related to R. sphae roides, Paracoccus denitrificans, and is lethal to R. sphaeroides. We suggest that toxicity is due to the ability of Rho' to form inactive h eteromers with the chromosomally encoded intact Rho, We localized a mi nimal amino acid sequence within Rho which appears to be critical for its toxic effect and which we believe may be involved in protein-prote in interactions, This region was previously reported to be highly cons erved and unique among various Rho proteins, The lethality of rho' in R. sphaeroides together with our inability to obtain a null mutation i n rho suggests that Rho dependent transcription termination is essenti al in R. sphaeroides. This is analogous to what is observed for gram-n egative E. coli and contrasts with what is observed for gram-positive Bacillus subtilis. The genetic region surrounding the R. sphaeroides r ho gene has been determined and found to be different compared with th ose of other bacterial species, rho is preceded by orf1, which encodes a putative integral membrane protein possibly involved in cytochrome formation or functioning, The gene downstream of rho is homologous to thdF, whose product is involved in thiophene and furan oxidation.