IMPLICATIONS OF RIBOSOMAL-RNA OPERON COPY NUMBER AND RIBOSOME CONTENTIN THE MARINE OLIGOTROPHIC ULTRAMICROBACTERIUM SPHINGOMONAS SP. STRAIN RB2256

Sphingomonas sp, strain RB2256 is a representative of the dominant cla ss of ultramicrobacteria that are present in marine oligotrophic water s. In this study we examined the rRNA copy number and ribosome content of RB2256 to identify factors that may be associated with the relativ ely low rate of growth exhibited by the organism. It was found that RB 2256 contains a single copy of the rRNA operon, in contrast to Vibrio spp,, which contain more than eight copies. The maximum number of ribo somes per cell was observed during mid-log phase; however, this maximu m content was low compared to those of faster-growing, heterotrophic b acteria (approximately 8% of the maximum ribosome content of Escherich ia coli with a growth rate of 1.5 h(-1)), The low number of ribosomes per cell appears to correlate with the low rate of growth (0.16 to 0.1 8 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 concent ration of ribosomes than E. coli (approximately double that off. coli with a growth rate of 1.5 h(-1)). Ribosome numbers reached maximum lev els during mid-log-phase growth but decreased rapidly to 10% of maximu m during late log phase through 7 days of starvation. The cells in lat e log phase and at the onset of starvation displayed an immediate resp onse to a sudden addition of excess glucose (3 mM). This result demons trates that a ribosome content 10% of maximum is sufficient to allow c ells to immediately respond to nutrient upshift and achieve maximum ra tes of growth, These data indicate that the bulk of the ribosome pool is not required for protein synthesis and that ribosomes are not the l imiting 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 a nd that they may be characteristics typical of oligotrophic ultramicro bacteria.