Hundreds of flagellar basal bodies cover the cell surface of the endosymbiotic bacterium Buchnera aphidicola sp. strain APS

Abstract

Buchnera aphidicola is the endosymbiotic bacterium of the pea aphid. Due to its small genome size, Buchnera lacks many essential genes for autogenous life but obtains nutrients from the host. Although the Buchnera cell is nonmotile, it retains clusters of flagellar genes that lack the late genes necessary for motility, including the flagellin gene. In this study, we show that the flagellar genes are actually transcribed and translated and that the Buchnera cell surface is covered with hundreds of hook-basal-body (HBB) complexes. The abundance of HBB complexes suggests a role other than motility. We discuss the possibility that the HBB complex may serve as a protein transporter not only for the flagellar proteins but also for other proteins to maintain the symbiotic system.

FIG. 1.

Organization of the flagellar genes on the chromosome of Buchnera aphidicola sp. strain APS. The genome size is 640,681 bp and holds 583 genes. Flagellar genes are densely packed in three clusters on the genome. Arrows indicate the directions of transcription.

FIG. 2.

Detection of flagellar gene expression by RT-PCR. Transcripts of 26 flagellar genes were amplified by RT-PCR with the specific primers listed in the text and electrophoresed on 2% agarose gel. +, PCR after RT; −, PCR without RT. M, molecular markers of 100-bp ladder. OmpF is the positive control.

FIG. 3.

Two-dimensional electrophoretic profile of Buchnera proteins. Buchnera cells isolated from bacteriocytes were lysed in rehydration buffer, and the aliquot was applied onto an Immobiline DryStrip (pI 4 to 7). After isoelectric focusing, followed by SDS-12.5% PAGE, peptide mass fingerprinting analysis by using MALDI-TOF/MS was performed. The largest spot was MopA, a GroEL-like 60-kDa chaperonin. Numbers: 1, FlgE; 2, FlgG; 3, DnaK; 4, RpsA; 5, MopA, 6, TufB; 7, OmpF-like protein.

FIG. 4.

Electron micrograph of the negatively stained Buchnera. A Buchnera whole cell shows that the periphery is covered with HBBs. Insets: A and B, enlarged views of the peripheral area of the cell surface; C and D, typical HBB-like particles are further enlarged. Buchnera organisms isolated from the bacteriocyte were stained with 1% sodium phosphotungstate. The specimen was observed by transmission electron microscopy. The scale bar for the main image is 500 nm; the scale bars for insets A and B is 100 nm and for insets C and D is 20 nm.