It has been discovered, surprisingly, that the novel amino acid substitutions and/or amino acid deletions of the carboxyl terminus of the serine acetyltransferase lead to a diminution within the cysteine sensitivity whereas at the same time allowing sufficient enzymic exercise to be retained. It is feasible to cut back the cysteine sensitivity of the serine acetyltransferase in vivo by producing, by the use of expression vectors, antisense RNAs that are complementary to an outlined area of the 3' coding strand of the native or remodeled cysE gene. The gene for serine acetyltransferase has already been cloned and the amino acid sequence which is deduced from the DNA sequence is understood (Denk, D. and Bock, A. 1987, J. Gen. Microbiol. In addition, O-acetylserine sulfhydrylase B (cysM) is ready to make the most of thiosulfate as a sulfur source (Sirko, A. et al., 1987, J. Gen. Microbiol. Methods for introducing mutations at specific positions within a DNA fragment are recognized and are described, for example, in the following publications: Sarkar, G., Sommer, S. S., 1990, BioTechniques 8: 404-407 describe site-particular mutagenesis using PCR; Ausubel, F. M. et al., 1987, pp. Another methodology of producing suggestions-resistant cysE alleles consists in combining different point mutations which lead to suggestions resistance, thereby giving rise to a number of mutants possessing new properties.

For that reason, the feedback-resistant cysE alleles are preferably integrated into the genome as single copies using customary strategies. Strains which include cysteine-sensitive proteins, for example prokaryotes or yeasts, are used as host strains. Since, in precept, cysteine metabolism proceeds by way of the same metabolic route, which is understood per se, in all microorganisms, and the strategies to be used for preparing the novel strains are well-known, for example from customary textbooks, and applicable to all microorganisms, novel strains might be ready from any microorganisms in any way. The invention also pertains to the preparation of L-cysteine, or of merchandise which are derived from L-cysteine, by way of cultivating novel microorganisms. The invention additionally relates to microorganisms which include the feedback-resistant cysE alleles. The present invention moreover pertains to DNA sequences which encode novel serine acetyltransferases. The coding sequences that are present on the vector are advantageously linked to regulatory components which are required for expressing the coding sequences to the desired extent. Sequences which encode selective markers and/or reporter genes are also ideally current on the expression vector along with the regulatory components.

An additional improve in the cysteine yield could be achieved by additionally overexpressing the sulfate-lowering enzymes (encoded by the genes cysD, C, H, G, I and J) and the sulfhydrating enzymes (encoded by the genes cysK and cysM). The formation of L-cysteine itself is catalyzed by two O-acetylserine sulfhydrylase isoenzymes (EC 4.2.99.8), encoded by the genes cysK (O-acetylserine sulfhydrylase A) and cysM (O-acetylserine sulfhydrylase B), a response through which O-acetylserine functions as a β-alanyl donor and H2 S as a β-alanyl acceptor (Kredich, N. M. and G. M. Tomkins 1966, J. Biol. The catalytic exercise of the totally different serine acetyltransferase enzymes is determined in the presence and N-Acetyl-L-Cysteine 98% bulk pricing absence of L-cysteine, and the inhibitor constant, Ki, is ascertained from this (Kredich and Tomkins, J. Biol. 1.1×10-6 M was determined in the presence of 0.1 mM acetyl-coenzyme A and 1 mM L-serine (Kredich, N. M. 1971 and Tomkins G. M. 1966, J. Biol.

The novel serine acetyltransferases ideally have an inhibitor constant, Ki, of from 0.005 to 2.3 mM within the presence of 1 mM L-serine and 0.1 mM acetyl-CoA, where serine acetyltransferases having not less than one mutation ideally possess an inhibitor constant, Ki, of from 0.015 to 2.3 mM within the presence of 1 mM L-serine and 0.1 mM acetyl-CoA, whereas serine acetyltransferases having at the very least one carboxyterminal deletion ideally exhibit an inhibitor constant, Ki, of from 0.005 to 0.03 mM within the presence of 1 mM L-serine and 0.1 mM acetyl-CoA. Strategies for integrating genes into the chromosome utilizing vectors whose origins of replication have been removed are state of the art (Winans et al., 1985; J. Bacteriol. It is a part of the state of the art to block or modify gene exercise in a particular manner by way of so-known as reverse genetics using antisense RNA (Inouye, 1988, Gene 72: 25-34). Antisense RNA is the transcription product of the DNA strand which is complementary to the strand encoding the protein. The beginning DNA fragment, encompassing, for instance, the wild-sort cysE gene, is recombined on a vector utilizing identified customary techniques for preparing recombinant DNA. The DNA of the wild-sort cysE gene, or a cysE gene which has been inactivated by mutation, or a cysE gene which has been mutated and which already encodes a feedback-resistant serine acetyltransferase, is ideally used because the starting materials for the mutagenesis.