Characterisation and Mutagenesis Study of An Alternative Sigma Factor Gene () from Reveal Its Central Role in Papaya Dieback Disease.

The alternative sigma (σ) factor E, RpoE or HrpL, has been reported to be involved in stress- and pathogenicity-related transcription initiation in and many other Gram-negative bacteria, including spp. and spp. A previous study identified the / transcript as one of the significant differentially expressed genes (DEGs) during early infection in papaya and those data serve as the basis of the current project. Here, the full coding DNA sequence (CDS) of from ( was determined to be 549 bp long, and it encoded a 21.3 kDa HrpL protein that possessed two highly conserved sigma-70 (σ) motifs-σR2 and σR4. Nucleotide sequence alignment revealed the from shared high sequence similarity to / from (83%), (81%), and (80%). Phylogenetics analysis indicated from to be monophyletic with s/s from , , and . Structural analysis postulated that the 's alternative σ factor was non-transmembranic and was an extracytoplasmic function (ECF) protein-characteristics shared by other σ factors in different bacterial species. Notably, the protein-protein interaction (PPI) study through molecular docking suggested the σ factor could be possibly inhibited by an anti-σ. Finally, a knockout of in (Δ) resulted in avirulence in four-month-old papaya plants. These findings have revealed that the is a necessary element in pathogenicity and also predicted that the gene can be inhibited by an anti-σ.