The packaging signal of Xanthomonas integrative filamentous phages

Abstract

Unlike Ff, the packaging signal (PS) and the mechanism of integrative filamentous phage assembly remain largely unknown. Here we revived two Inoviridae prophage sequences, {phi}Lf2 and {phi}Lf-UK, as infectious virions lysogenize black rot pathogen Xanthomonas campestris pv. campestris. {phi}Lf2 and {phi}Lf-UK genomes consist of 6,363 and 6,062 nucleotides and share 85.8% and 98.7% identity with {phi}Lf, respectively. To explore their assembly, we first identified 20-26- nucleotide long PS sequences of 10 Xanthomonas phages. These PS consist of a DNA hairpin with the consensus GGX(A/-)CCG(C/T)G sequence in the stem and C/T nucleotides in the loop, both of which are conserved and essential for PS activity. In contrast to Ff, the 5 to 3 orientation of PS sequence is not conserved or critical for their competence. This is the first report to offer insights into the structure and function of integrative phage PS, revealing the diversity of filamentous phage encapsidation.