Ribonuclease production by Aspergillus species

Ribonuclease production by Aspergillus flavipes, A. sulphureus And A. fischeri in semisynthetic medium, after 24-144 hours at 30§C under shaking, was studied. After cultivation, the medium was separated from micelia by filtration and the resultant solution was used as enzymatic extract. The highest amount of biomass and RNase was obtained after 96 hours of cultivation. The enzymes produced by three species presented similar characteristics, with optimum temperature at 55§C and two peaks of activity at pH 4.5 and 7.0. A. flavipes RNases were more sensitive to temperatue: 50(per cent) of the initial activity was lost after 1 hour at 70§C. After this heat treatment, RNase of A. sulphureus lost 30(per cent) of this activity and that of A. fischeri only 16(per cent). The nucleotides released by enzimatic hydrolysis of RNA were separated by ion exchange chromatography in a AG-1X8-formiate column and identified by paper chromatography. This procedure indicated that the raw enzymatic extract of Aspergillus flavipes is able to hydrolyze RNA, releasing 3'-nucleotides monophosphate at pH 4.5 and 3'and 5'-nucleotides monophosphate at pH 7.0 and 8.5. This result suggests that this strain produces two different types of RNase, one acidic and other alcaline, with different specificities

Retrorregulación: un mecanismo postranscripcional que controla la expresión genética / Retroregulation: a postranscriptional mechanism that controls gene expresión

La síntese de la integrasa del bacteriófago lambda se regula de manera diferencial durante las distintas etapas del ciclo infectivo. La expresión del gene int está sujeta a regulación postranscripcional por la región sib, localizada después del gene int en el DNA de lambda. La región sib regula negativamente la expresión del promotor pL. En este proceso, denominado retroinhibición, participan además de la RNasaIII, la polinucleótido fosforilasa y probablemente la RNasaII de E. coli. Por otro lado, la terminación de la transcripción en el sitio tI estimula la síntesis de la integrasa cuando esta se expresa a partir del promotor pI. A este proceso se le ha llamado retroestimulación. la región sib se sobrepone con el sitio terminador de la transcripción tI, sin embargo, la retrorregulación y la terminación de la transcripción son procesos independientes