This review highlights the recent investigations of class IV polyhydroxyalkanoate (PHA) synthases, the newest classification of PHA synthases. Class IV synthases are prevalent in organisms of the Bacillus genus and are composed of a catalytic subunit PhaC (approximately 40 kDa), which has a PhaC box sequence ([GS]-X-C-X-[GA]-G) at the active site, and a second subunit PhaR (approximately 20 kDa). The representative PHA-producing Bacillus strains are Bacillus megaterium and Bacillus cereus; the nucleotide sequence of phaC and the genetic organization of the PHA biosynthesis gene locus are somewhat different between these two strains. It is generally considered that class IV synthases favor short-chain-length monomers such as 3-hydroxybutyrate (C4) and 3-hydroxyvalerate (C5) for polymerization, but can polymerize some unusual monomers as minor components. In Escherichia coli expressing PhaRC from B. cereus YB-4, the biosynthesized PHA undergoes synthase-catalyzed alcoholytic cleavage using endogenous and exogenous alcohols. This alcoholysis is thought to be shared among class IV synthases, and this reaction is useful not only for the regulation of PHA molecular weight but also for the modification of the PHA carboxy terminus. The novel properties of class IV synthases will open up the possibility for the design of new PHA materials.

Polyhydroxyalkanoate (PHA)-producing Bacillus strains possess class IV PHA synthases composed of two subunit types, namely, PhaR and PhaC. In the present study, PHA synthases from Bacillus megaterium NBRC15308(T) (PhaRC(Bm)), B. cereus YB-4 (PhaRC(YB4)), and hybrids (PhaR(Bm)C(YB4) and PhaR(YB4)C(Bm)) were expressed in Escherichia coli JM109 to characterize the molecular weight of the synthesized poly(3-hydroxybutyrate) [P(3HB)]. PhaRC(Bm) synthesized P(3HB) with a relatively high molecular weight (M(n) = 890 × 10(3)) during 72 h of cultivation, whereas PhaRC(YB4) synthesized low-molecular-weight P(3HB) (M(n) = 20 × 10(3)). The molecular weight of P(3HB) synthesized by PhaRC(YB4) decreased with increasing culture time and temperature. This time-dependent behavior was observed for hybrid synthase PhaR(Bm)C(YB4), but not for PhaR(YB4)C(Bm). These results suggest that the molecular weight change is caused by the PhaC(YB4) subunit. The homology between PhaCs from B. megaterium and B. cereus YB-4 is 71% (amino acid identity); however, PhaC(YB4) was found to have a previously unknown effect on the molecular weight of the P(3HB) synthesized in E. coli.

The biofilm degradation of Aggregatibacter actinomycetemcomitans is essential as a complete periodontal disease therapy, and here we show the effects of potential probiotic bacteria such as Lactobacillus spp. for the biofilm of several serotypes of A. actinomycetemcomitans strains. Eight of the 13 species showed the competent biofilm degradation of ≥ 90% reduction in biofilm values in A. actinomycetemcomitans Y4 (serotype b) as well as four of the seven species for the biofilm of A. actinomycetemcomitans OMZ 534 (serotype e). In contrast, the probiotic bacteria did not have a big impact for the degradation of A. actinomycetemcomitans SUNY 75 (serotype a) biofilm. The dispersed A. actinomycetemcomitans Y4 cells through the biofilm detachment were still viable and plausible factors for the biofilm degradation were not due to the lactic acid and low pH conditions. The three enzymes, protease, lipase, and amylase may be responsible for the biofilm degradation; in particular, lipase was the most effective enzyme for the biofilm degradation of A. actinomycetemcomitans Y4 along with the protease activity which should be also important for the other serotypes. Remarkable lipase enzyme activities were detected from some of the potential probiotics and a supporting result using a lipase inhibitor presented corroborating evidence that lipase activity is one of the contributing factors for biofilm degradation outside of the protease which is also another possible factor for the biofilm of the other serotype of A. actinomycetemcomitans strains. On the other hand, the biofilm of A. actinomycetemcomitans SUNY 75 (serotype a) was not powerfully degraded by the lipase enzyme because the lipase inhibitor was slightly functional for only two of potential probiotics.

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