SUMMARY: Cell mass, the average number of nuclei/cell and the content of RNA and DNA were studied in Salmonella typhimurium during balanced (steady state) growth in different media. These quantities could be described as exponential functions of the growth rates afforded by the various media a t a given temperature. The size and chemical composition characteristic of a given medium were not influenced by the temperature of cultivation. Thus, under conditions of balanced growth, this organism exists in one of a large number of possible stable physiological states.The variations in mass/cell are due to changes in the number of nuclei/cell as well as in mass/nucleus. An increase in the number of ribonucleoprotein particles at higher growth rates could, it appears, largely account for the increase in mass/nucleus. Calculations indicate that the rate of protein synthesis per unit RNA is nearly the same a t all growth rates.

SUMMARY : When cultures of Salmonella typhimurium undergoing balanced growth are shifted from one medium to another, a definite pattern of rate changes is observed. Shifts from a low to a high growth rate result in a strict succession of events: RNA synthesis is immediately affected and its rate rapidly increases to that characteristic of the new medium; the increase in optical density shows a lag of a few minutes before the new rate is attained; DNA synthesis and cell division, on the other hand, continue at the old rate for appreciable periods of time and then abruptly shift to the new rates. The times at which these shifts take place are, at 37', invariably 20 and 70 min., regardless of the actual growth rates before and after the shift. This rate maintenance effect on DNA synthesis and cell division is discussed in terms of specific rate-controlling mechanisms.In the preceding paper (Schaechter, Maalnre & Kjeldgaard, 1959) we showed that the size and chemical composition of Salmonella typhimurium vary exponentially with the growth rate afforded by different media. The present paper describes the transition from one state of balanced growth to another. The terms 'balanced growth' and 'cell' as used by us, have been defined in the preceding paper. Two types of experiments were performed, both involving precisely timed changes of medium at constant temperatures; one going to a higher growth rate (shift up), and the other to a lower rate (shijt d m ) .The shift up, exemplified by going from a glucose salt medium to nutrient broth, is characterized by an orderly dissociation of the main synthetic activities. Upon addition of broth, the rates of synthesis of ribonucleic acid (RNA) and of total mass immediately increase, whereas deoxyribonucleic acid (DNA) synthesis and cell division continue at the old rate for considerable periods of time. The new state of balanced growth is reached when, after about 70 min. (at 37O), the rate of cell division abruptly changes from thepreshift value to the definitive broth rate. We interpret this 'rate maintenance effect ' as evidence for the existence of separate rate-controlling mechanisms.The shqt down is rapidly effected by filtration of, say, a nutrient broth culture and resuspension of the cells in simple defined medium. It is characterized by a period during which cell division and DNA synthesis continue in the absence of net synthesis of RNA and total mass. By the time RNA and mass begin to increase the cells have already approached the size and the composition typical of balanced growth in the simpler medium. This behaviour

2'-5'-Oligoadenylate synthetase was among the first interferon-induced antiviral enzymes to be discovered. This family of enzymes plays an important role in the mechanisms of action of interferon antiviral activity, but is also involved in other cellular processes such as apoptosis and growth control. We have reviewed the function and genomic structure of this class of at least nine proteins. By studying the recently available data in the human genome database and the human Expressed Sequence Tag database, we have been able to build a comprehensive picture of the 2'-5'-oligoadenylate synthetase gene family and its precise location on chromosome 12. Chromosomal localization as well as the intron/exon structure of all four genes has been established and an overview of the splice variant forms of the 2'-5'-oligoadenylate synthetases arising from expression of the four genes is presented. Alignments of the human 2'-5'-oligoadenylate synthetase sequences with non-human 2'-5'-oligoadenylate synthetase sequences suggest that the exon structure and several amino acid sequence motifs have been conserved during evolution.

Escheridzia cdi contains an abundant DNA binding protein, HU, [l] with a monomeric mol. wt -10 000 which is readily isolated by affinity chromatography on DNA-cellulose. Similar protein fractions, showing immunological cross reactivity with the E. coli protein, have been isolated from ~a~rno~~~la ~p~~rnu~urn (Prigent, J. R.-Y., unpubli~ed), 3aci~l~ss~~~i~is (unpublished) and from cyanobacteria [2]. HU is a basic protein which bears similarities to histone H2b in its amino acid composition. Recent in vitro experiments have shown that HU can introduce negative superhelical turns in a relaxed DNA in the presence of a nicking-closing enzyme in a similar fashion to the 4 core histones [3]. A pure preparation of HU, homogeneous on SDS-polyac~lamide gel electrophoresis revealed heterogeneities at specific positions along the polypeptide chain, suggesting the existence of two closely related variants of the HU fractions [4].Two proteins, NSl and NS2, similar to HU, have been isolated from native 30 S ribosomal subunits of E. cob [S]. The complete amino acid sequences of these two proteins were determined [6] and found to be highly homologous with the N-terminal sequences of HU. This suggested that the NS proteins are identical with our HU fraction. However, we have shown [7] that only a minor fraction of HU (-10%) is bound to the ribosomes. The majority of HU protein is associated with the bacterial nucleoid isolated at low salt [7].Here we report that the majority of native HU is a Materials and methodsProtein HU isolated from E. coii cells W3350 was purified as in ]I] by DNA-cellulose chromatography and Sephadex G-100 filtration. After dialysis against buffer containing 10 mM Tris-HCl, (pH 7.5), 1 mM EDTA and 60 mM NaCl, the protein was adsorbed to a preequilibrated Pl 1 Whatman phosphocellulose column and eluted with a linear gradient from 60-600 mM NaCl in the same buffer. Proteins were analysed either by conventions IO-25% SDSpolyac~l~ide gel electrophoresis [ 1] or by acidurea-Triton polyacrylamide gel electrophoresis. This later electrophoresis was performed in a 7.5% polyacrylamide gel containing 25% acetic acid (gel system no. 9 of [8]). The system was modified by the addition of urea (2.5 M) and Triton X-100 (1%) to the separation gel, the spacer gel and the sample buffer. The gel was photopolymerised with riboflav~. Electrophoresis was performed overnight at 120-I 50 V in the cold room. The dimethyl suberimidate crosslinking was performed according to [9]. The protein samples, dissolved in or dialysed against 0.2 M triethanolamine-HCl buffer (pH 8.2) at 200 Erg/ml final cont. were treated for 3 h at room temperature with 2 m&ml dimethyl suberimidate. After the incubation period, samples were precipitated by 50% trichloroacetic acid, dissolved in SDS-sample buffer and analysed by SDS-polyacrylamide gel electrophoresis. The N-terminal sequencing was done in a Beckman automatic protein sequencer in the labora-297

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