The operon or operon is a classic bacterial gene cluster, which codes for proteins K, A, J and L of the large ribosomal subunit, as well as proteins B ( subunit) and C (′ subunit) of RNA polymerase. In the early 1990s, the operon was obtained as a 2.6 kbp DNA fragment (In-2.6) by random cloning of DNA from periwinkle plants infected with the Poona (India) strain of the huanglongbing agent, later named ‘ () Liberibacter asiaticus’. DNA from periwinkle plants infected with the Nelspruit strain (South Africa) of ‘ L. africanus’ was amplified with a primer pair designed from In-2.6 and yielded, after cloning and sequencing, a 1.7 kbp DNA fragment (AS-1.7) of the operon of ‘ L. africanus’. The operon of the American liberibacter, as well as the three upstream genes (, , ), have now also been obtained by the technique of chromosome walking and extend over 4673 bp, comprising the following genes: , , , , , , and . The sequence of the operon was also determined for a Brazilian strain of ‘ L. asiaticus’, from to (3025 bp), and was found to share 99 % identity with the corresponding operon sequences of an Indian and a Japanese strain. Finally, the operon sequence of ‘ L. africanus’ was extended from 1673 bp ( to ) to 3013 bp ( to ), making it possible to compare the operon sequences of the African, Asian and American liberibacters over a length of ∼3000 bp, from to . While ‘. L. africanus’ and ‘. L. asiaticus’ shared 81.2 % sequence identity, the percentage for ‘. L. americanus’ and ‘. L. africanus’ was only 72.2 %, and identity for ‘. L. americanus’ and ‘. L. asiaticus’ was only 71.4 %. The ∼3000 bp sequence was also used to construct a phylogenetic tree, and this tree was found to be identical to the known 16S rRNA gene sequence-based tree. These results confirm earlier findings that ‘. L. americanus’ is a distinct liberibacter, more distantly related to ‘. L. africanus’ and ‘. L. asiaticus’ than ‘. L. africanus’ is to ‘. L. asiaticus’. The dates of speciation have also been estimated.

Keyword(s): HLB, huanglongbing

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