ABSTRACT
Microbial degradation of natural rubber and synthetic poly(cis-1,4-isoprene) is expected to become an alternative treatment system for waste from poly(cis-1,4-isoprene) products including scrap tires. Nocardia farcinica NBRC 15,532, a gram-positive rubber-degrading bacterium, can utilize poly(cis-1,4-isoprene) as the sole source of carbon and energy to produce oligo-isoprene metabolites containing aldehyde and keto end groups. A homology-based search of the genome revealed a gene encoding a latex-clearing protein (Lcp). Gene disruption analysis indicated that this gene is essential for the utilization of poly(cis-1,4-isoprene) in this strain. Further analysis of the genome sequence identified aldehyde dehydrogenase (ALDH) genes as potential candidates for oxidative degradation of oligo-isoprene aldehydes. Based on the enzymatic activity of the ALDH candidates, NF2_RS14000 and NF2_RS14385 may be involved in the degradation of oligo-isoprene aldehydes. Analysis of the reaction products revealed that these ALDHs oxidized tri- to penta-isoprene aldehydes, which were generated by the reaction of Lcp. Based on the inability of ALDH gene deletion mutants, we concluded that NF2_RS14000 is mainly involved in the utilization of poly(cis-1,4-isoprene) and the oxidative degradation of oligo-isoprene aldehydes in Nocardia farcinica NBRC 15,532.
ABSTRACT
The microbial degradation of synthetic and natural poly(cis-1,4-isoprene) rubber is expected to become an alternative treatment technique for waste from poly(cis-1,4-isoprene) products, such as scrap tires. A gram-positive rubber-degrading bacterium, Rhodococcus sp. strain RDE2, was isolated from the waste of a rubber-processing factory in Vietnam. This strain grew on natural rubber as a sole source of carbon and energy and produced oligo-isoprenoid metabolites containing aldehyde groups from poly(cis-1,4-isoprene). To identify the genes responsible for poly(cis-1,4-isoprene) degradation, the complete genome sequence of this strain was determined. The complete genome sequence consists of a 5,715,406 bp chromosome and 6 plasmids (GenBank accession numbers AP025186.1 to AP025192.1) with an average GC content of 67.9%. The genome contains 5358 protein-coding sequences and 12 and 68 copies of rRNA and tRNA genes, respectively. Based on genome sequence analysis, the lcp gene (RDE2_08,770), responsible for the initial step of poly(cis-1,4-isoprene) degradation, was identified. The gene product obtained from Escherichia coli depolymerizes poly(cis-1,4-isoprene) to low-molecular-weight oligo-isoprenoids. The transcription of this gene is activated during the utilization of poly(cis-1,4-isoprene) in strain RDE2. The lcpR gene (RDE2_08,760), which encodes a putative transcriptional regulator, is located upstream of lcp. The lcpR gene product recognizes the promoter region of lcp. When the lcpR gene is deleted, the constitutive transcription of lcp is observed. Thus, it is inferred that the LcpR negatively regulates lcp transcription. These results strongly suggest that the lcp and lcpR genes are involved in poly(cis-1,4-isoprene) utilization in strain RDE2.
