This phylogenetic tree was constructed using 16S rRNA gene sequences. The Proteo

Question

This phylogenetic tree was constructed using 16S rRNA gene sequences. The Proteobacterial clade (Liberibacter to Haemophilus) is gram negative. The Firmicutes clade (Bacillus to Mycoplasma) is gram positive.

Give an evolutionary explanation for how Mycoplasma can cluster with the Firmicutes in this tree but stain gram negative.

Candidatus Liberibacter asiaticus Sinorhizobium meliloti a-proteobacteria Rickettsia prowazeki Ralstonia solanacearum Neisseria meningitidis Wigglesworthia brevipalpis 3-proteobacteria Buchnera sp APS y-proteobacteria Escherichia coli Candidatus Phlomobacter fragariae Haemophilus influenzae Bacillus subtilis Firmicutes Candidatus Phytoplasma asteris OY Candidatus Phytoplasma asteris AY-WB Candidatus Phytoplasma australiense Candidatus Phytoplasma mali Ureaplasma urealyticum Mycoplasma pneumoniae Mycoplasma genitalium Mollicutes Chlamydia trachomatis

Explanation / Answer

Firmiculates are gram-positive bacteria that have a thick peptodiglycan layer in cell wall with no outer membrane. They have a low guanine + cytosine content in genomic DNA (less than 50%).

The 16S rRNA-based phylogenetic classification of bacteria has become universally accepted to classify bacteria. In Gram-positive bacteria, the 16S rRNA sequences share conserved oligonucleotide signatures. Mycoplasma are clustered in Firmiculates because of similarity in their 16S rRNA sequences. Further, 16S rRNA-based trees are similar to phylogenetic trees based on other popular markers. These markers include ribosomal proteins, DNA gyrase subunit GyrB, RNA polymerase subunits, etc.

The nucleoside diphosphate kinase (NdK) gene, ubiquitous gene expressing essential metabolic activity is absent both in Mycoplasma and other Firmiculates. Firmiuclates have sterols in their cell wall, making the cell wall more rigid.

However, Mycoplasma differ from Firmiculates in the fact that they lack a cell wall and intracytoplasmic membranes. They, however, have sterols in their cell membranes, as other Firmiculates. These sterols regulate membrane fluidity. As they lack a cell wall, they will be gram negative. However, gram negative bacteria do have a thin cell wall with limited peptidoglycan. However, Mycoplasma don’t have any cell wall. Hence, it is opined that they originated from gram positive Firmiculates but lost their cell walls. They are classified as Firmiculates due to low G+C content and 16S rRNA analysis.

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