| Abstract: | The biogenesis of 30 S and 50 S ribosomal subunits in exponentially growing Escherichia coli has been studied by following the rate of appearance of pulse-labelled ribosomal proteins on mature subunits. Cells were pulse-labelled for two minutes and for three and a half minutes with radioactive leucine. Ribosomal proteins were extracted and purified by chromatography on carboxymethyl cellulose and analysed by bidimensional gel electrophoresis. All 30 S proteins and most of the 50 S proteins were thus prepared and their radioactivity counted: unequal labelling was obtained. 30 S and 50 S proteins were ordered according to increasing specific radioactivity at both time pulses. The incorporation was greater at three and a half minutes than at two minutes. No major difference in the order at the two labelling times was observed.Only two classes of proteins can be defined in the 30 S and the 50 S subunits, namely early and late proteins. In each class a gradual increase in the radioactivity is apparent from the poorly labelled to the highly labelled proteins. This suggests a definite order of addition.Early 30 S proteins: S17, S16, S15, S19, S18, S8, S4, S20, S10, S6, S9, S12, S7.Late 30 S proteins: S5, S3, S2, S14, S11, S13, S1, S21.Early 50 S proteins: L22, L20, L21, L4, L13, L16, L3, L23, L18, L24, L28, L17, L19, L29, L32, L5, L15, L2, L30, L27.Late 50 S proteins: L25, L11, L7, L12, L1, L9, L8, L10, L33, L14, L6.This order is discussed taking into account the pool size of the proteins measured in the same conditions of cell culture. |
