GROWTH-RATE REGULATION OF RIBOSOMAL-RNA CONTENT OF A MARINE SYNECHOCOCCUS (CYANOBACTERIUM) STRAIN

The relationship between growth rate and rRNA content in a marine Syne chococcus strain was examined. A combination of flow cytometry and who le-cell hybridization with fluorescently labeled 16S rRNA-targeted oli gonucleotide probes was used to measure the rRNA content of Synechococ cus strain WH8101 cells grown at a range of light-limited growth rates . The sensitivity of this approach was sufficient for the analysis of rRNA even in very slowly growing Synechococcus cells (mu = 0.15 day(-1 )). The relationship between growth rate and cellular rRNA content com prised three phases: (i) at low grow-th rates (<similar to 0.7 day(-1) ), rRNA cell(-1) remained approximately constant; (ii) at intermediate rates (similar to 0.7 - 1.6 day(-1)), rRNA cell(-1) increased proport ionally with growth rate; and (iii) at the highest, light-saturated ra tes (> similar to 1.6 day(-1)), rRNA cell(-1) dropped abruptly, Total cellular RNA (as measured with the nucleic acid stain SYBR Green II) w as well correlated with the probe-based measure of rRNA and varied in a similar manner with growth rate. Mean cell volume and rRNA concentra tion (amount of rRNA per cubic micrometer) were related to growth rate in a manner similar to rRNA cell(-1), although the overall magnitude of change in both cases was reduced, These patterns are hypothesized t o reflect an approximately linear increase in ribosome efficiency with increasing growth rate, which is consistent with the prevailing proka ryotic model at low growth rates. Taken together, these results suppor t the notion that measurements of cellular rRNA content might be usefu l for estimating in situ growth rates in natural Synechococcus populat ions.