biostudies-literatureMordaka PMBill & Melinda Gates FoundationUKRI | Biotechnology and Biological Sciences Research CouncilBill &amp; Melinda Gates FoundationHuman Frontier Science ProgramHuman Frontier Science Program (HFSP)Biotechnology and Biological Sciences Research Councile2506263123https://www.ebi.ac.uk/biostudies/studies/S-EPMC12799115biostudies-literatureUnknown123(2)Genome scale engineering has enabled codon compression of the universal genetic code to eliminate seven codons in <i><i>Escherichia coli</i></i>, but to allow more radical schemes for codon compression and reassignment to be tested at genome scale, while avoiding significant technical challenges, smaller, simpler genetic systems are needed. Here, we report a recoding scheme for the 205 kb <i>Chlamydomonas reinhardtii</i> chloroplast genome, in which two stop codons and one or more of the codons for arginine, glycine, isoleucine, leucine, and serine, all of which have two cognate transfer RNAs (tRNAs), are absent, compressing the genetic code to 51 codons. Several recoding strategies were tested on the essential <i>rpoA</i> gene, encoding a subunit of the chloroplast RNA polymerase. A definProceedings of the National Academy of Sciences of the United States of America&lt;i&gt;Chlamydomonas&lt;/i&gt; chloroplast genes tolerate compression of the genetic code to just 51 codons.PMC12799115OPP1144RGP0058/2020BB/W003538/1BB/R01860X/1Mordaka PMCui JJackson HOHolzer ASmith AGClouston KPurton Sfalse&lt;i&gt;Chlamydomonas&lt;/i&gt; chloroplast genes tolerate compression of the genetic code to just 51 codons.Genome scale engineering has enabled codon compression of the universal genetic code to eliminate seven codons in <i><i>Escherichia coli</i></i>, but to allow more radical schemes for codon compression and reassignment to be tested at genome scale, while avoiding significant technical challenges, smaller, simpler genetic systems are needed. Here, we report a recoding scheme for the 205 kb <i>Chlamydomonas reinhardtii</i> chloroplast genome, in which two stop codons and one or more of the codons for arginine, glycine, isoleucine, leucine, and serine, all of which have two cognate transfer RNAs (tRNAs), are absent, compressing the genetic code to 51 codons. Several recoding strategies were tested on the essential <i>rpoA</i> gene, encoding a subunit of the chloroplast RNA polymerase. A defin2026-01-01T00:00:00Z2026 Jan2026-06-06T15:23:35.337Z2026-06-01T03:11:04.986ZS-EPMC127991154149381110.1073/pnas.2506263123