This review examines polyadenylation of bacterial mRNA, a process long dismissed as eukaryotic-specific but now recognized as widespread in prokaryotes. Despite early biochemical evidence from the 1970s, bacterial poly(A) RNA was initially skeptically received due to methodological challenges, low detection levels, and unclear physiological significance. Using improved isolation techniques, researchers demonstrated that 15-40% of newly synthesized mRNA in species like Bacillus brevis and B. subtilis carries poly(A) tracts of 10-80 nucleotides at their 3' ends. These polyadenylated mRNAs were identified as typical size for mRNA, associated with polysomes, and highly active in protein synthesis. Molecular characterization of E. coli mRNAs revealed two classes of polyadenylated transcripts with different termination sites, suggesting polyadenylation occurs as a post-transcriptional processing event. The enzyme poly(A) polymerase (PAP), encoded by the pcnB gene, catalyzes poly(A) synthesis and functions similarly to eukaryotic PAPs. Evidence suggests mRNA polyadenylation may modulate stability through competition with degradative exonucleases. Multiple PAP activities and complex interactions with nucleolytic pathways indicate bacterial mRNA polyadenylation involves intricate enzymatic machinery, though its precise functional role remains unclear.

Key findings

• Polyadenylation is extensive in bacterial mRNA (15-40% of newly synthesized mRNA), with poly(A) tracts of 10-80 nucleotides, indicating it is not exclusively eukaryotic
• Two classes of polyadenylated E. coli mRNAs exist with different 3' polyadenylation sites, suggesting post-transcriptional processing independent of transcription termination
• Bacterial poly(A) polymerase (PAP) is encoded by pcnB gene; at least two PAP activities exist (PAP I and PAP II) with overlapping functions
• Polyadenylation appears to modulate mRNA stability through competition between poly(A) polymerase and 3'-exonucleases (RNase II, PNPase) at mRNA 3' ends
• Polyadenylated mRNAs are highly active in translation and associated with polysomes, comprising diverse mRNA sequences rather than a restricted subpopulation