Biologically active isoforms of CobB sirtuin deacetylase in Salmonella enterica and Erwinia amylovora

Sirtuins are NAD(+)-dependent protein deacylases that are conserved in all domains of life and are involved in diverse cellular processes, including control of gene expression and central metabolism. Eukaryotic sirtuins have N-terminal extensions that have been linked to protein multimerization and cellular localization. Here the first evidence of sirtuin isoforms in bacteria is reported. The enterobacterium Salmonella enterica synthesizes two isoforms of CobB sirtuin, a shorter 236-amino-acid isoform (here CobB(S)) and a longer 273-amino-acid isoform (here CobB(L)). The N-terminal 37-amino-acid extension of CobB(L) is amphipathic, containing 18 basic amino acids (12 of which are Arg) and 13 hydrophobic ones; both isoforms were active in vivo and in vitro. Northern blot and transcription start site analyses revealed that cobB is primarily expressed as two monocistronic cobB mRNAs from two transcription start sites, one of which was mapped within the neighboring ycfX gene and the other of which was located within cobB. Additionally, a low-abundance ycfX-cobB bicistronic mRNA was observed which could encode up to three proteins (YcfX, CobB(L), and CobB(S)). CobB(L) isoforms are common within the family Enterobacteriaceae, but species of the genus Erwinia (including the plant pathogen Erwinia amylovora) encode only the CobB(L) isoform. The CobB(L) isoform from E. amylovora restored growth of as S. enterica cobB mutant strain on low acetate.