A group II intron has invaded the genus Azotobacter and is inserted within the termination codon of the essential groEL gene.

A group II intron that was previously identified within Azotobacter vinelandii by polymerase chain reac-tion with consensus primers has been completely sequenced, together with its flanking exons. In contrast to other bacterial members of group II, which are associated with mobile or other presumably non-essential DNA, the A. vinelandii intron is inserted within the termination codon of the groEL coding sequence, which it changes from UAA to UAG. Both the host gene and the intron appear to be functional as (i) the ribozyme component of the intron self-splices in vitro and (ii) both intron-carrying and intronless versions of the single-copy groEL gene from A. vinelandii complement groEL mutations in Escherichia coli. Moreover, analysis of nucleotide substitutions within and around a closely related intron sequence that is present at the same site in Azotobacter chroococcum provides indirect evidence of intron transposition posterior to the divergence of the two Azotobacter taxa. Somewhat surprisingly, however, analyses of RNA extracted from cells that had or had not undergone a heat shock show that the bulk of groEL transcripts end within the first 140 nucleotides of the intron. These findings are discussed in the light of our current knowledge of the biochemistry of group II introns.