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Applied and Environmental Microbiology, November 1998, p. 4433-4438, Vol. 64, No. 11
School of Microbiology and Immunology, The
University of New South Wales, Sydney, 2052 New South Wales,
Australia
Received 2 July 1998/Accepted 31 August 1998
Sphingomonas sp. strain RB2256 is a representative of
the dominant class of ultramicrobacteria that are present in marine oligotrophic waters. In this study we examined the rRNA copy number and
ribosome content of RB2256 to identify factors that may be associated
with the relatively low rate of growth exhibited by the organism. It
was found that RB2256 contains a single copy of the rRNA operon, in
contrast to Vibrio spp., which contain more than eight
copies. The maximum number of ribosomes per cell was observed during
mid-log phase; however, this maximum content was low compared to those
of faster-growing, heterotrophic bacteria (approximately 8% of the
maximum ribosome content of Escherichia coli with a growth
rate of 1.5 h
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Copyright © 1998, American Society for Microbiology. All rights reserved.
Implications of rRNA Operon Copy Number and Ribosome Content in
the Marine Oligotrophic Ultramicrobacterium Sphingomonas
sp. Strain RB2256
1). The low number of ribosomes per cell
appears to correlate with the low rate of growth (0.16 to 0.18 h
1) and the presence of a single copy of the rRNA operon.
However, on the basis of cell volume, RB2256 appears to have a higher
concentration of ribosomes than E. coli (approximately
double that of E. coli with a growth rate of 1.5 h
1). Ribosome numbers reached maximum levels
during mid-log-phase growth but decreased rapidly to 10% of maximum
during late log phase through 7 days of starvation. The cells in late
log phase and at the onset of starvation displayed an immediate
response to a sudden addition of excess glucose (3 mM). This result
demonstrates that a ribosome content 10% of
maximum is sufficient to allow cells to immediately respond to nutrient
upshift and achieve maximum rates of growth. These data indicate
that the bulk of the ribosome pool is not required for protein
synthesis and that ribosomes are not the limiting factor contributing
to a low rate of growth. Our findings show that the regulation of
ribosome content, the number of ribosomes per cell, and growth rate
responses in RB2256 are fundamentally different from those
characteristics in fast-growing heterotrophs like E. coli
and that they may be characteristics typical of oligotrophic ultramicrobacteria.
*
Corresponding author. Mailing address: School of
Microbiology and Immunology, The University of New South Wales, Sydney,
2052 NSW, Australia. Phone: (61) 2-9385 3516. Fax: (61) 2-9385 1591. E-mail: r.cavicchioli{at}unsw.edu.au.
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